fix(sandbox): canonicalize HTTP request-targets before L7 policy evaluation

[johntmyers]

Summary

• The L7 REST proxy evaluates OPA path rules against the raw HTTP request-target and forwards the raw header block byte-for-byte to the upstream. The upstream normalizes .., %2e%2e, and // before dispatching, so a compromised agent inside the sandbox can bypass any path-based allow rule — e.g. GET /public/../secret matches /public/** at the proxy but the upstream serves /secret.
• The inference-routing detection (normalize_inference_path) had the same normalization gap: it only stripped scheme+authority and preserved dot-segments verbatim, so the same class of payload could also dodge inference routing.
• This PR introduces a single URL-path canonicalizer used at every L7 enforcement site. The canonical path is both the input to OPA and the bytes written onto the wire — closing the parser-differential between the policy engine and the upstream.

Related Issue

OS-99

Changes

• crates/openshell-sandbox/src/l7/path.rs (new)
  • canonicalize_request_target(target, opts) -> Result<(CanonicalPath, Option<String> /* raw query */), CanonicalizeError>.
  • Percent-decodes (uppercase hex on re-emit), resolves . / .. per RFC 3986 5.2.4 (rejects underflow rather than silently clamping to /), collapses doubled slashes, strips trailing ;params, handles origin-form and absolute-form targets, enforces a 4 KiB length bound, rejects fragments / raw non-ASCII / control bytes / encoded slashes (unless explicitly opted in per-endpoint via allow_encoded_slash).
  • Returns a rewritten flag so callers know whether the outbound request line needs to be rebuilt.
• crates/openshell-sandbox/src/l7/rest.rs
  • parse_http_request canonicalizes the request-target before returning the L7Request. If the canonical form differs from the raw input, the request line in raw_header is rebuilt via rewrite_request_line_target so the upstream dispatches on the same bytes the policy evaluated.
  • parse_query_params is now pub(crate) so proxy.rs can reuse it.
• crates/openshell-sandbox/src/proxy.rs
  • The forward (plain HTTP proxy) L7 evaluation site canonicalizes the request-target. Non-canonical input is denied with 400 Bad Request and an OCSF NetworkActivity Fail event instead of being silently passed through.
  • normalize_inference_path is now a thin wrapper over the canonicalizer (falls back to the raw path on canonicalization error so the normal forward path can reject properly).
• crates/openshell-sandbox/data/sandbox-policy.rego
  • Comment documenting the invariant: input.request.path is pre-canonicalized, so rules must not attempt defensive matching against .. or %2e%2e.
• architecture/sandbox.md
  • Adds the new l7/path.rs module to the component table.

Testing

• cargo test -p openshell-sandbox --lib — 513 tests pass, including 24 new tests in l7::path::tests (dot segments, percent-encoded dot segments, double-slash collapse, encoded-slash reject/opt-in, null/control byte rejection, fragment rejection, absolute-form stripping, legitimate %20 / %40 / %C3%A9 round-tripping, mixed-case percent normalization, length bound, non-ASCII rejection, query-suffix separation, rewritten-flag semantics).
• cargo test -p openshell-sandbox — 518 tests across all targets pass.
• mise run pre-commit — passes (lint, format, license headers, rust tests).
• Regression tests for the documented bypass payloads:
  • /public/../secret → /secret
  • /public/%2e%2e/secret → /secret
  • /public/%2E%2E%2Fsecret → EncodedSlashNotAllowed
  • //public/../secret → /secret
  • /public/./../secret → /secret
  • /public/%00/secret → NullOrControlByte
  • /.. → TraversalAboveRoot
• Legitimate traffic continues to pass: /files/hello%20world.txt, /users/caf%C3%A9, /v1/chat/completions?stream=true, paths with : and @ in segments.

Trade-offs

• %2F inside a segment is rejected by default. Operators who need it for artifact-registry-style APIs can opt in per-endpoint via allow_encoded_slash: true (the plumbing is in place — endpoint config wire-up can land in a follow-up once there's a consumer).
• Non-canonical requests are always hard-rejected regardless of EnforcementMode::Audit — audit mode is for policy-tuning, not for tolerating protocol violations.
• %20 and similar legitimate percent-encoded bytes are preserved in the canonical form (only unreserved pchars get decoded), so policy patterns that currently match /files/hello%20world.txt continue to work.

Checklist

• Conventional Commits format
• No secrets or credentials in diff
• Scoped to the issue at hand (no unrelated refactors)
• Architecture and rego documentation updated

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