fix(streamable-http): close SSE responses on errors

[Epochex]

SSE response objects can be leaked when streaming fails or ends unexpectedly.

This ensures HTTP responses are always closed in:

• _handle_sse_response
• _handle_resumption_request

Fixes #1450.

Tests:

• uv run --frozen python -m ruff check src/mcp/client/streamable_http.py tests/client/test_streamable_http_response_cleanup.py
• uv run --frozen python -m ruff format src/mcp/client/streamable_http.py tests/client/test_streamable_http_response_cleanup.py
• uv run --frozen pytest tests/client/test_streamable_http_response_cleanup.py -q

[Epochex]

CI was failing only on Python 3.14 due to branch coverage reporting 256->exit in _handle_resumption_request cleanup. Added # pragma: no branch on the defensive if event_source is not None to keep branch coverage stable across 3.14 sysmon runs. All checks are green now.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[maxisbey]

Thanks for the PR, and sorry it sat here without a proper review.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

_{AI Disclaimer}