test: update SSE tests to use StreamingASGITransport instead of uvicorn for server testing

[bjoaquinc]

This implements fixes to the StreamingASGITransport to properly handle SSE streaming and disconnect signaling. The key changes were:

• Added disconnect signaling via receive() to fix SSE hanging
• Fixed memory stream cleanup in SseServerTransport to eliminate ResourceWarnings
• Handle duplicate http.response.start with NoopASGI
• Fixed global sse-starlette quirk
• Update all tests in tests/server/test_sse_security.py and tests/shared/test_sse.py to use transports instead of real servers with uvicorn

Motivation and Context

Partially resolves #857

How Has This Been Tested?

Since I updated the testing methodology I made sure to run the test in batches of 30 (to check for race conditions). Using the command:

passed=0; failed=0; for i in {1..30}; do echo "=== Run $i/30 ===" && uv run --resolution="$RESOLUTION" pytest "$FILE_PATH" -q && passed=$((passed+1)) || failed=$((failed+1)); done; echo "==== SUMMARY: $passed passed, $failed failed out of 30 runs ===="

for the following resolutions and files:

• tests/server/test_sse_security.py for lowest-direct and highest
• tests/shared/test_sse.py for lowest-direct and highest

All tests passed:

Breaking Changes

None. StreamingASGITransport is only meant for testing. The only change I made to SseServerTransport was to properly clean up sse_stream_reader which is best practice and doesn't break functionality. The modified tests all function as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• 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

Additional context

[bjoaquinc]

Done with all updates and ready for a review.

[maxisbey]

Related: #1728 (v2 test client improvements)

_{AI Disclaimer}