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 gracefully during teardown

I also modified one test in tests/shared/test_sse.py to validate this methodology. I'll be updating all tests after validation.

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 20 (to check for race conditions). After the updates I was able to get it working for the modified test.

Breaking Changes

None. StreamingASGITransport is only meant for testing. Also before this PR, it was not implemented anywhere in the codebase. 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 test_request_context_propagation functions 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