#3249 fix(server): handle POST body disconnects gracefully

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.