test(ingestion): /mcp's JWT validator and resource_metadata; narrow comments

PR #183 review (comment 15474):
- TestBuildMuxAnswers503ToAJWTOnMCPWhileTheIssuerIsUnresolved: building
  /mcp's middleware with a nil validator turns it red.
- TestBuildMuxMCP401CarriesResourceMetadataInWWWAuthenticate: building it
  with resourceMetadataURL "" turns it red.
- main.go, routes.go and the testMux comment now say exactly what is
  untested: main's restAuth fields and public map passed to buildMux, and
  ListenAndServe (hyperguild#182). Stale public-route list in the
  TestBuildMuxDoesNotWrapPublicRoutes comment corrected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/cmd/server/routes.go

@@ -41,8 +41,9 @@ type restAuth struct {
 // wrapped in reg's middleware. Other public handlers are registered as given,
 // and only for patterns on the publicRoutes allowlist. It returns an
 // http.Handler, not the *ServeMux, so nothing can register a route on the
-// result. What stays untested is main's own line: a handler main built around
-// this one before ListenAndServe would not be seen by these tests.
+// result. What stays untested is main's call site: the restAuth fields and
+// the public map main passes in, and ListenAndServe serving this handler
+// unchanged (hyperguild#182). The tests build their own arguments.
 func buildMux(
 	auth restAuth, h *api.Handler, mcpHandler http.Handler, reg *metrics.Registry, public map[string]http.Handler,
 ) (http.Handler, error) {