Preserve queued SSE progress notifications

[lorenzozanee]

Fixes #6349

Issue: #6349

Problem: A POST-SSE request can lose progress notifications when a final response arrives at nearly the same time.

Root cause: The progress and final-response channels are both buffered, so select can choose the final response while a progress notification remains queued.

Change: Drain queued progress notifications before writing the final SSE response.

Tests:

• Added a regression test for a queued progress notification and final response.
• Added unit coverage for draining pending progress notifications.