fix(client): bound empty GET stream retries

[daleselaji-dev]

Problem

StreamableHTTPTransport.handle_get_stream() can reconnect forever when a server returns a valid empty SSE stream and closes it cleanly. The loop's MAX_RECONNECTION_ATTEMPTS bound is never reached because every empty close resets the attempt counter.

Root Cause

The transport treated every normal SSE iterator completion as a productive stream, even when no SSE event had been received.

Solution

Track whether the stream delivered an event. Reset the retry counter only after a productive stream; count an empty clean close against the existing retry budget.

Changes

• Bound empty GET/SSE stream retries at MAX_RECONNECTION_ATTEMPTS.
• Preserve existing retry reset behavior for streams that deliver events.
• Add a regression test with an empty 200 text/event-stream response.

Testing

• Baseline: existing header and exhausted-reconnection tests ? 9 passed.
• PYTHONPATH=src;. py -3 -m pytest --import-mode=importlib tests/client/test_streamable_http.py -q ? 29 passed.
• py -3 -m ruff format --check src/mcp/client/streamable_http.py tests/client/test_streamable_http.py ? passed.
• py -3 -m ruff check src/mcp/client/streamable_http.py tests/client/test_streamable_http.py ? passed.
• py -3 -m pyright --pythonpath D:\Python310\python.exe src/mcp/client/streamable_http.py tests/client/test_streamable_http.py ? passed.
• git diff --check ? passed.

Compatibility / Risk

This only changes behavior for a cleanly closed stream that delivered zero events. Productive SSE streams continue to reset the retry budget and reconnect as before. The change prevents unbounded background polling against endpoints that terminate without sending data.

Notes for Reviewer

The regression uses the real transport with httpx2.MockTransport, a valid empty SSE response, and a patched zero-delay sleep; it asserts exactly the configured retry budget is consumed.

Linked Issue

Fixes #3257

[cubic-dev-ai]

All reported issues were addressed across 2 files

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