fix: isolate streamable HTTP POST errors

[pragnyanramtha]

Summary

• convert non-404 POST HTTP status failures into request-scoped JSON-RPC errors for JSON-RPC requests
• avoid synthesizing responses for notification/no-id messages
• add a regression test proving one failed concurrent POST request no longer cancels an unrelated sibling request

Fixes#2604.

This targets v1.x because the released 1.x branch still raises through response.raise_for_status() for POST HTTP errors. main already has equivalent scoped >=400 handling; this PR backports that behavior and adds the concurrency regression coverage for the reported failure mode.

Validation

• uv run --frozen pytest tests/shared/test_streamable_http.py::test_streamable_http_client_http_error_does_not_cancel_concurrent_request
• uv run --frozen pytest tests/shared/test_streamable_http.py -> 52 passed
• uv run --frozen pytest tests/shared/test_streamable_http.py::test_streamable_http_client_http_error_does_not_cancel_concurrent_request tests/shared/test_streamable_http.py::test_streamable_http_client_session_termination tests/shared/test_streamable_http.py::test_streamable_http_client_session_termination_204 tests/client/test_notification_response.py::test_non_compliant_notification_response -> 4 passed
• uv run --frozen ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv sync --frozen --all-extras --dev
• uv run --frozen pyright -> 0 errors
• git diff --check

[Copilot]

Pull request overview

Backports main’s request-scoped handling of HTTP POST failures for the Streamable HTTP transport in the v1.x line, preventing a single request’s non-2xx POST response from escaping the per-request task and tearing down concurrent sibling requests.

Changes:

• Convert POST HTTP >=400 (non-404) responses into request-scoped JSON-RPC errors for JSON-RPC requests (and avoid synthesizing responses for notifications).
• Add a concurrency regression test ensuring one failing POST does not cancel an unrelated in-flight request.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/mcp/client/streamable_http.py	Scopes POST HTTP error handling to the originating JSON-RPC request instead of raising through the shared task group.
tests/shared/test_streamable_http.py	Adds a regression test covering concurrent requests where one receives an HTTP 400 and the other still completes successfully.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Kludex]

You've opened a duplicated pull request, please search opened PRs before creating new ones. Duplicated from #1998.