fix(server): handle POST body disconnects gracefully

[Linxiushen]

Summary

• handle Starlette ClientDisconnect while reading a Streamable HTTP POST body as a normal client-side event
• return without attempting to send an HTTP 500 response to a client that has already disconnected
• keep the disconnect out of the internal session stream because no JSON-RPC message was parsed or dispatched
• add direct ASGI regression coverage for the partial-body disconnect path

Fixes #1648.

Verification

• uv run --frozen pytest tests/server/test_streamable_http_router.py -q (4 passed)
• uv run --frozen ruff check src/mcp/server/streamable_http.py tests/server/test_streamable_http_router.py
• uv run --frozen ruff format --check src/mcp/server/streamable_http.py tests/server/test_streamable_http_router.py
• uv run --frozen pyright src/mcp/server/streamable_http.py tests/server/test_streamable_http_router.py (0 errors)

The full scripts/test -n 0 run collected 5,592 tests but could not complete on this Windows host because unrelated stdio interaction tests were unable to spawn child Python processes (PermissionError: [WinError 5]).

AI assistance was used during investigation and implementation. I reviewed the final diff and take responsibility for the change.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #1648.

If a maintainer would like this change as a PR from you, they'll assign you to #1648 and this PR will reopen automatically — there's nothing more you need to do. (If you opened the issue, this PR already shows up on its timeline.)

There's no need to open a new PR — this one will be reopened. While it's closed, please push any updates as new commits rather than force-pushing, since GitHub can't reopen a PR whose branch has been rewritten.

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