feat: MCP App validator (library + CLI) — reference implementation for #673

[RyanAlberts]

What

A working reference implementation of the official MCP App validator proposed in #673, opened as a draft to make that discussion concrete. Happy to restructure any part of it to maintainer preference (naming, location, scope).

New export @modelcontextprotocol/ext-apps/validator + mcp-app-validator bin:

• Requirements catalogue (src/validator/rules.ts): every check is backed by a rule with a stable id (APP-001…), the normative MUST/SHOULD sentence it enforces, and the spec section — MUST → error, SHOULD → warning. The direction field (app/server/host) reserves the same id space for host-side rules, so the platform-conformance work in Set up public MCP Apps platform tests #674 and this validator can share one catalogue instead of drifting apart.
• Static checks (no rendering): connects with the io.modelcontextprotocol/ui extension capability advertised, then validates tools/list / resources/list / resources/read: ui:// scheme, text/html;profile=mcp-app mimeType, text-or-blob content, HTML sanity, tool-referenced resources resolving, _meta.ui shapes via the SDK's generated Zod schemas, CSP origin well-formedness, visibility values, deprecated flat-key usage.
• Behavioral checks: loads the app HTML under a minimal Playwright mock host speaking the postMessage JSON-RPC protocol; checks the ui/initialize handshake, appCapabilities presence, initialized notification, JSON-RPC well-formedness / known methods, size-changed emission, tools/list answered when the tools capability is declared, and display-mode discipline. Playwright is resolved lazily — static validation runs without it, and behavioral rules report themselves as skipped when it's absent.
• CLI: mcp-app-validator <url> / --stdio <cmd…> / --html <file>, --json for CI, exit codes 0 (clean) / 1 (errors) / 2 (usage or connection failure).

Validation

• bun test src: 298 pass (14 new validator tests using the existing McpServer + InMemoryTransport pattern)
• Ran the CLI against all 22 Node example servers in this repo: all pass; pdf-server correctly draws the SHOULD-level APP-104 warning (no ui/notifications/size-changed observed)
• Negative case: a non-app HTML document fails APP-100 with exit code 1

Open questions (mirrored on #673)

1. Naming/placement: separate bin vs folding into a future create-mcp-app CLI (Add official CLI scaffolding tool for MCP Apps #354)?
2. Should the behavioral harness exercise a real tool-call path via a --tool flag instead of a synthetic ui/notifications/tool-input?
3. Ship a JSON catalogue export for Set up public MCP Apps platform tests #674/third-party harnesses now, or after rule ids stabilize?

Aware of the SDK v2 migration (#702/#710) — SDK contact is contained to two small modules, and I'm glad to rebase onto the v2 tree if preferred.

Refs #673

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