fix: surface stateful HTTP session crash cause

[he-yufeng]

Summary

• surface the original stateful session app.run() exception when a POST fails after the session stream closes
• clean up the per-request memory stream on that failure path
• avoid a secondary ClosedResourceError when reporting the failure back to an already closed writer

Fixes #2741.

To verify

• python -m pytest tests\server\test_streamable_http_manager.py -q
• python -m ruff check src\mcp\server\streamable_http.py src\mcp\server\streamable_http_manager.py tests\server\test_streamable_http_manager.py
• python -m ruff format --check src\mcp\server\streamable_http.py src\mcp\server\streamable_http_manager.py tests\server\test_streamable_http_manager.py
• git diff --check -- src\mcp\server\streamable_http.py src\mcp\server\streamable_http_manager.py tests\server\test_streamable_http_manager.py

[he-yufeng]

Rebased this on current main and carried the crash-cause reporting over the updated streamable HTTP code.

Validation run locally on Windows:

python -m py_compile src\mcp\server\streamable_http.py src\mcp\server\streamable_http_manager.py tests\server\test_streamable_http_manager.py
uv run pytest tests/server/test_streamable_http_manager.py -q -k "session_crash_cause or post_error_message" --basetemp .tmp\pytest-2742-20260610 -p no:cacheprovider
# 2 passed, 20 deselected

git diff --check upstream/main..HEAD

The error log lines in that targeted pytest run are expected: the new regression test deliberately crashes the session runner and asserts that the JSON response reports the original session exception instead of only the downstream closed-stream error.

[he-yufeng]

Closing this. Since I opened it, main has reworked the streamable-HTTP POST error path in the opposite direction — it logs the exception server-side and returns a generic "Error handling POST request" message rather than surfacing the underlying error to the client. This PR's whole point was to surface the session crash cause, which now cuts against that deliberate choice, so it's cleaner to drop it than to force a conflicting reconcile. Thanks for the refactor.