fix: isolate streamable HTTP request errors

[he-yufeng]

Summary

• catch per-request failures in the streamable HTTP writer before they escape the shared task group
• return a JSON-RPC error for the failed request while keeping the writer usable for later requests
• add a regression test that sends a failing request followed by a successful one on the same transport

Closes #2604

To verify

• uv run pytest tests/shared/test_streamable_http.py -q -k "request_error_does_not_close_writer"
• uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run python -m py_compile src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• git diff --check

[he-yufeng]

Updated the branch after the CI coverage failure. The missing streamable_http.py exception path is now covered by a notification-error regression test. Local checks: targeted pytest passed; full tests/shared/test_streamable_http.py passed (63 passed); coverage report for src/mcp/client/streamable_http.py no longer lists line 478 as missing; ruff check, ruff format --check, pyright, py_compile, and git diff --check all passed. CI is running again on f218dc8.

[he-yufeng]

Updated once more after the Python 3.14 strict coverage run exposed a branch-coverage partial in the new regression test itself. The new test now follows the existing anyio.fail_after pragma style. Local checks: targeted pytest passed; full tests/shared/test_streamable_http.py passed (63 passed); coverage report for tests/shared/test_streamable_http.py is 100.00%; ruff check, ruff format --check, pyright, py_compile, and git diff --check all passed. CI is running again on f092047.

[he-yufeng]

CI is green now on f092047: all matrix test jobs, pre-commit, readme-snippets, conformance, and all-green passed.

[he-yufeng]

Rebased this onto current upstream/main and resolved the StreamableHTTP in-process transport test refactor conflict.

Kept the request-error isolation behavior and migrated the surrounding tests to the current in-process test setup without bringing back the old socket test style.

Validated locally on Windows:

• PYTHONUTF8=1 uv run pytest tests/shared/test_streamable_http.py -q -k "request_error_does_not_close_writer or notification_error_still_closes_writer or session_persistence" (3 passed)
• PYTHONUTF8=1 uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• PYTHONUTF8=1 uv run ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• PYTHONUTF8=1 uv run pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• PYTHONUTF8=1 uv run python -m py_compile src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• git diff --check upstream/main..HEAD

[he-yufeng]

Follow-up after the matrix failure: the request-error isolation was catching every exception from the request task, which also converted OAuth flow errors into JSON-RPC errors. I narrowed that handler to httpx.HTTPError, so transport/request failures are still isolated for JSON-RPC requests, while auth flow errors keep propagating as OAuthFlowError/OAuthRegistrationError.

Validated locally on Windows:

• PYTHONUTF8=1 uv run pytest tests/shared/test_streamable_http.py tests/interaction/auth/test_authorize_token.py tests/interaction/auth/test_discovery.py -q -k "request_error_does_not_close_writer or notification_error_still_closes_writer or session_persistence or mismatched_state_on_the_callback or authorize_error_on_the_callback or registration_endpoint or prm_with_a_mismatched_resource" (7 passed)
• PYTHONUTF8=1 uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/auth/test_authorize_token.py tests/interaction/auth/test_discovery.py
• PYTHONUTF8=1 uv run ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/auth/test_authorize_token.py tests/interaction/auth/test_discovery.py
• PYTHONUTF8=1 uv run pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/auth/test_authorize_token.py tests/interaction/auth/test_discovery.py
• PYTHONUTF8=1 uv run python -m py_compile src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/auth/test_authorize_token.py tests/interaction/auth/test_discovery.py
• git diff --check upstream/main..HEAD