test(aspnetcore): cover RequireAuthorization() on the MCP endpoint across session modes

[ump45nose]

Summary

Adds end-to-end coverage for protecting the MCP endpoint itself with RequireAuthorization().

AuthorizeAttributeTests already covers tool-level [Authorize]: the request reaches the MCP handler and the tool collection is filtered per user. The endpoint-level challenge has different requirements and had no coverage — it must happen before the handler runs, it must hold for every request in an established session, and it must work in each session mode without making the OAuth protected-resource metadata document unreachable.

New file: tests/ModelContextProtocol.AspNetCore.Tests/MapMcpAuthorizationTests.cs

• Initialize_WithoutCredentials_IsChallenged — the initial initialize POST is rejected with 401 in Stateful, StatefulForInitializeClients, and Stateless.
• Initialize_WithCredentials_Succeeds — 200, and a session id is minted in the session-based modes but not in Stateless (which keeps the endpoint free of server state).
• StatefulSession_FollowUpRequests_RequireCredentials — an authorized initialize followed by an authorized tools/list on the same session works, while the same Mcp-Session-Id sent without credentials is rejected with 401. The session id identifies a session; it must not authenticate the caller.
• ResourceMetadata_RemainsReachable_WhileTheMcpEndpointIsChallenged — with MapMcp("/mcp").RequireAuthorization() and the SDK's McpAuthenticationHandler configured, GET /.well-known/oauth-protected-resource/mcp is served without credentials, and the 401 challenge advertises resource_metadata="…/.well-known/oauth-protected-resource/mcp" so a client can still discover the authorization server from the challenge alone. The endpoint is mounted at /mcp here because that is the deployment shape where the metadata document mirrors the resource path.

The authentication scheme is a test-only header-based handler, so the challenged and authorized paths are both exercised without standing up a token issuer.

No product code changes.

Verification

dotnet build tests/ModelContextProtocol.AspNetCore.Tests succeeds for net10.0, net9.0, and net8.0 with 0 warnings.

dotnet test -c Release -f net10.0 (SDK 10.0.101) on this branch:

• ModelContextProtocol.AspNetCore.Tests: 602 passed, 55 skipped, 0 failed
• ModelContextProtocol.Tests: 2359 passed, 5 skipped, 0 failed
• MapMcpAuthorizationTests on its own: 8 passed, 0 failed

Only the .NET 10 runtime is installed on my machine, so the net8.0/net9.0 legs are compile-verified rather than executed here. ModelContextProtocol.Analyzers.Tests targets net9.0 only and is unaffected by this change.

Built and tested locally with -p:NuGetAudit=false: Microsoft.Build.Tasks.Git 8.0.0 raises NU1902 on this repository today, which reproduces on unmodified main and is unrelated to this change.

Related to #1785, where endpoint-level RequireAuthorization() combined with session mode was the configuration under investigation.

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.