Handle client disconnects while reading POST body

[linhongyu510]

Fixes #1648.

Motivation and Context

This PR handles client disconnects while reading Streamable HTTP POST request bodies without reporting them as server errors.

When Starlette raises ClientDisconnect while reading the POST body, the current handler falls through to the generic exception path. That logs an error and attempts to return a 500 response, even though the client has already disconnected.

How Has This Been Tested?

env -u PYTHONHOME -u PYTHONPATH uv run pytest tests/shared/test_streamable_http.py -k 'post_client_disconnect_is_not_reported_as_500 or transport_reports_stream_closure'
env -u PYTHONHOME -u PYTHONPATH uv run pyright src/mcp/server/streamable_http.py tests/shared/test_streamable_http.py
env -u PYTHONHOME -u PYTHONPATH uv run ruff format src/mcp/server/streamable_http.py tests/shared/test_streamable_http.py --check
env -u PYTHONHOME -u PYTHONPATH uv run ruff check src/mcp/server/streamable_http.py tests/shared/test_streamable_http.py

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have disclosed AI assistance and can explain the change in my own words
• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #1648.

If a maintainer assigns you to #1648, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.