fix: 405 for unsupported MCP route methods + survive invalid WebSocket close frames

[aryrabelo]

Symptom

Running a self-hosted Hub against real daemon/agent traffic, two failures showed up:

1. GET /agent-executions/:id/mcp (and DELETE) is not handled by the route, so it falls through to the SPA route render: a 200 with the app-shell HTML for an unknown execution id, or a 500 for a real one. An MCP client that probes the transport with a method other than POST (or a proxy/monitor that does a GET health check) reads the 500 as "server is dead" instead of a clean "method not supported, retry with POST".
2. The Hub process crashed outright with:

   RangeError [WS_ERR_INVALID_CLOSE_CODE]: invalid status code
       at Receiver.controlMessage (ws/lib/receiver.js)
   

   as an unhandled process.uncaught_exception, taking down every daemon connection on the instance, not just the one that sent the bad frame.

Cause

1. src/routes/agent-executions/$executionId/mcp.ts only declared a POST handler. TanStack Start has no fallback 405 for undeclared methods on a route that also matches page rendering, so GET/DELETE hit the SPA path instead.
2. ActiveDaemonRegistry.accept() (src/daemons/registry.ts) registers "message" and "close" listeners on a daemon's WebSocket but never "error". When a peer sends a close control frame carrying a status code the protocol forbids on the wire (ws's Receiver rejects reserved codes such as 1004-1006), ws emits error on that socket. Node's EventEmitter rethrows an unlistened error event as an uncaught exception, killing the whole process.

Fix

• Add explicit GET/DELETE handlers on the MCP route that return 405 with Allow: POST, matching the error body shape already used by handleExecutionCapabilities in the same module.
• Add an error listener in accept() that reports through the existing reportFailure/report pipeline. ws already closes the connection itself once the error fires, so the existing close handler still drives normal offline-presence cleanup — only that one daemon's socket is affected now.

How to reproduce / verify

• WebSocket crash: src/daemons/registry.test.ts — new test writes a raw close frame with status 1006 directly to the underlying TCP socket (bypassing ws's own close() validation, which already rejects invalid codes client-side) via a new DaemonRegistryHarness.sendInvalidClose() helper. Without the fix this crashes the vitest worker with the exact WS_ERR_INVALID_CLOSE_CODE seen in production; with the fix it asserts the failure is reported and the socket still closes cleanly.
• 405 route guard: src/mcp-route-method-guard.test.ts — calls the route's GET/DELETE handlers directly and asserts 405 + Allow: POST. Without the fix, Route.options.server.handlers.GET doesn't exist and the test throws handlers.GET is not a function.
• Manually verified against a built instance of this branch (embedded/pglite database, no DATABASE_URL): curl against /agent-executions/does-not-exist/mcp returns 405 with Allow: POST for both GET and DELETE, and /health still returns 200.

Not changed

Checked whether @modelcontextprotocol/sdk could be bumped to support MCP protocol version 2026-07-28 (seen advertised by an external client). The installed version, 1.30.0, is also the latest version published to npm, and its SUPPORTED_PROTOCOL_VERSIONS tops out at 2025-11-25. No published SDK version supports 2026-07-28 yet, so there is nothing to bump here — that ceiling is upstream in @modelcontextprotocol/sdk.

Test commands run

npm run typecheck:node
npm run typecheck:start
npx oxlint --config oxlint.json --tsconfig tsconfig.json src/daemons/registry.ts src/daemons/test-utils/daemon-registry-harness.ts src/mcp-route-method-guard.test.ts
npx oxlint --config oxlint.json --tsconfig tsconfig.json src/routes/agent-executions/\$executionId/mcp.ts
npx vitest run src/daemons/registry.test.ts src/mcp-route-method-guard.test.ts
npm run build

Did not run the full npm test/npm run lint/npm run release:check suite (out of scope for this change; the e2e suite additionally requires Docker/Postgres via testcontainers).