Make Streamable HTTP status mapping deterministic

[halter73]

Summary

• remove the 250 ms deferred-header grace timer and its lifecycle/logging code
• wait for the actual first JSON-RPC message on 2026-07-28 requests before committing headers, so SEP-2575 error-to-HTTP status mapping is deterministic under load
• preserve eager response-header flushing for legacy protocol revisions
• remove the new public callback overload and carry the ASP.NET Core response-start hook through an internal request-context feature
• buffer SSE priming/polling writes until the first JSON-RPC message on the deferred path, keeping response start exactly-once under the transport lock

This intentionally leaves quiet 2026-07-28 handlers subject to the caller's HttpClient timeout unless they emit an inline message such as progress. It does not add StartResponseAsync; a Streamable HTTP-only API on RequestContext<T> needs a separate design.

Validation

• dotnet build passes with 0 warnings and 0 errors
• focused header/status regressions pass on .NET 8, 9, and 10
• all in-repo test suites pass across their target frameworks
• the external DockerEverythingServerTests.Sampling_Sse_EverythingServer remains environment-blocked because the configured running server does not expose trigger-sampling-request; the restored npm everything-server integration tests pass

Note

This pull request description was generated with GitHub Copilot.