Preserve the resource path in the default resource_metadata challenge

[sherter]

Expected Behavior

For a resource server mounted below the root, the default WWW-Authenticate challenge would ideally
follow RFC 9728 §3.1 and insert the well-known segment between host and resource path:

WWW-Authenticate: Bearer resource_metadata="https://host/.well-known/oauth-protected-resource/mcp"

Current Behavior

BearerTokenAuthenticationEntryPoint.getResourceMetadataParameter replaces the entire path, so every
resource is advertised at the root document:

WWW-Authenticate: Bearer resource_metadata="https://host/.well-known/oauth-protected-resource"

Context

OAuth2ProtectedResourceMetadataFilter already supports the suffixed form — it matches
/.well-known/oauth-protected-resource/** and resolveResourceIdentifier strips the well-known
segment, returning "resource": "https://host/mcp". Aligning the entry point's default with what the
filter already serves would make the two consistent out of the box.

#18542 added setResourceMetadataParameterResolver for exactly this case, so nothing is blocked. But
the resolver a non-root deployment needs is the framework's own default plus the endpoint path, which
every such application ends up reimplementing — Spring AI MCP servers (endpoint defaults to /mcp)
hit it by default.

Would a default that derives the path from the request, falling back to current behaviour at the root,
be in scope?

Context: Spring Boot 4.1.0 / Spring AI 2.0.0, oauth2ResourceServer with protectedResourceMetadata,
server mounted at /mcp.

(Note: This report was drafted in parts by Opus 5)