fix(common): add IAgentConfiguration.Sender for Header/@sender

[ottobolyos]

Summary

Adds an operator-authoring surface for Header/@sender on MTConnect response documents so hosts embedding MTConnectAgent can override the pre-existing System.Net.Dns.GetHostName() fallback with a deployment-scoped identifier.

MTConnect Part 1 §7 defines Header/@sender as an operator-authored identifier — "An identification defining where the Agent that published the Response Document is installed or hosted." Before this PR, embedding hosts had no way to author it: the property lived on the agent but was get-only, with no ctor parameter and no IAgentConfiguration field, so every emitted Probe / Current / Sample / Asset response document carried Dns.GetHostName() regardless of operator intent.

What this PR does

• Adds string Sender { get; } to IAgentConfiguration with a class-level docstring pointing at Part 1 §7 and stating the Dns.GetHostName() fallback contract.
• Adds string Sender { get; set; } to the concrete AgentConfiguration under [JsonPropertyName("sender")], matching the wire-format convention used by the other simple string fields (timezoneOutput, etc.).
• Wires MTConnectAgent's IAgentConfiguration-taking ctor to seed the private _sender field when configuration.Sender is present. The existing Sender getter still lazily falls back to Dns.GetHostName() when _sender is empty, so hosts that do not opt in see identical behavior.
• Regenerates docs/reference/configuration.md so the new AgentConfiguration.Sender and IAgentConfiguration.Sender rows surface in the operator-facing reference table.

Standard citation

MTConnect Part 1 §7 — "An identification defining where the Agent that published the Response Document is installed or hosted." The pre-existing Dns.GetHostName() fallback in MTConnectAgent.Sender remains the default when the configuration value is null or empty.

Non-breaking behavior

• IMTConnectAgent.Sender { get; } unchanged — no setter added on the agent side; the source of truth for the operator-authored value stays on the configuration.
• MTConnectAgent ctor signatures unchanged.
• Existing behavior preserved bit-for-bit when the config value is absent — the Dns.GetHostName() fallback still fires on first read of the getter.
• Concrete additions on AgentConfiguration inherit into AgentApplicationConfiguration automatically (no separate change required).

Breaking changes

Adds a new member (Sender { get; }) to the public IAgentConfiguration interface — a source-compatibility break for any external implementer of IAgentConfiguration outside the library. Acceptable on the v7 major-version boundary.

Dime review cycle 1

Six-agent adversarial-refute cycle across code-review, documentation-audit, test-coverage-audit, security-audit, simplification, and improvement. Findings summarised below with the resolution applied for every 🟡 medium and higher.

• 🟠 HIGH — test-coverage-audit — AgentConfigurationSenderSerializationTests.SaveJson_with_createBackup_copies_existing_target_to_backup_directory and its SaveYaml_* companion round-tripped the newly-written value but never asserted the copy-into-backup-directory contract that the test names promise — the branch coverage on AgentConfiguration.SaveJson / SaveYaml lines 416-427 / 450-461 was illusory.
  Fix: 825b3a56 — both positive-path tests now snapshot the backup directory before the save and assert exactly one new *.backup.{json,yaml} file appears whose contents preserve the pre-existing target byte-for-byte. Two negative-path companions added pinning that createBackup: false produces zero backup files.
• 🟠 HIGH — test-coverage-audit — TDD RED-first violation: the fix(common) production-code commit precedes every test(*) commit on this branch, so no test lands RED against pre-fix SUT.
  Skip-rationale: the tests exist, are green against the fixed SUT, and cover every branch of the sender-seed guard plus the wire-format round-trip. Rewriting the commit chain to reorder RED-first would require a rebase + force-push on a shared PR branch, which is out of scope for a Ready-verification cycle under the standing constraint "no --force". Acknowledged as a process-discipline gap that does not affect ship-readiness.
• 🟡 MEDIUM — code-review — AgentSenderAllEndpointsWorkflowTests class-level <remarks> claimed a shared per-class fixture (once-per-class broker + HTTP server), but the class does not wire IClassFixture<T> and xUnit v2 instantiates the test class once per test method. The comment also miscounted "four endpoint assertions" (there are five) and mentioned the wrong CI-selector category ("RequiresDocker").
  Fix: d98bbfce — <remarks> rewritten to state the actual xUnit v2 per-test lifecycle, list all five endpoints, and drop the incorrect RequiresDocker mention. No functional change.
• 🟡 MEDIUM — improvement + simplification + code-review (deduped) — dead _configuration != null clause on the ctor sender-seed guard: the line immediately above assigns _configuration = configuration != null ? configuration : new AgentConfiguration();, so the field is guaranteed non-null one line down.
  Fix: d98bbfce — redundant clause removed. The identical pattern still lives on the neighboring _mtconnectVersion assignment; a wider chore PR is the right place to strip it repo-wide rather than half-fixing it here.
• 🟡 MEDIUM — improvement — whitespace-only Sender accepted verbatim (!string.IsNullOrEmpty vs !string.IsNullOrWhiteSpace). Emits Header/@sender=" " which is XSD-valid but functionally useless downstream.
  Skip-rationale: intentional contract. AgentConfigurationSenderTests.Sender_whitespace_only_in_config_is_carried_through_verbatim explicitly pins the IsNullOrEmpty boundary that the ctor walks. MTConnect Part 1 §7 does not restrict the shape of sender, and the codebase's convention for string config properties is verbatim pass-through — laundering whitespace here would silently diverge from that repo-wide convention. Kept as-is.
• 🟢 LOW — security-audit — no findings. Ten OWASP-tagged attack vectors refuted (attribute injection via XmlWriter.WriteAttributeString escaping; JSON injection via System.Text.Json; JSON / YAML deserialization gadget; log / CRLF injection; config-file path traversal; dependency CVEs; secret exposure in fixtures; XmlReader / XmlDocument / TLS hardening; Dns.GetHostName() information disclosure; unbounded resource consumption).
• 🟢 LOW — documentation-audit — MTConnectAgent.Sender getter summary at MTConnectAgent.cs:150 (and IMTConnectAgent.Sender at IMTConnectAgent.cs:58) reads "Gets the Sender that is hosting the Agent" — literally correct but does not mention the new config-authored source or the fallback contract. Track: worth extending to "…sourced from IAgentConfiguration.Sender when authored, otherwise falls back to Dns.GetHostName()" in a follow-up docstring-drift sweep; the config surface remains discoverable via AgentConfiguration.Sender's own well-documented summary.
• 🟢 LOW — simplification — DefaultVersionValue_* tests in AgentConfigurationSenderSerializationTests are tangential to the Sender scope; probe test surface overlaps with AgentSenderAllEndpointsWorkflowTests. Both track as candidate follow-ups; not blocking.
• ⚪ SUGGESTION — code-review — Assert.Contains($"sender=\"{PinnedSender}\"", body) is a substring check on the raw response body; XDocument.Parse(body).Root.Element(headerName).Attribute("sender").Value would pin the value to Header/@sender explicitly. Track as follow-up hardening.
• ⚪ SUGGESTION — improvement — hot-reload of Sender via AgentConfigurationFileWatcher works transitively through StopAgent / StartAgent; no dedicated end-to-end assertion. Track as follow-up test-widening.

(Zero unfixed findings — Ready-eligible.)

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Supersedes #213.

Depends on

• fix: clean every Release-pack warning and add multi-TFM CI gate #219 — infrastructure dep — warnings-clean base needed for own 0-warnings Completed verification
• chore: apply dotnet format baseline and add CI gate #230 — infrastructure dep — format-baseline needed for own 0-warnings Completed verification