Fix dropped progress frames in POST-SSE responses

[isaacgao4396]

Summary

• Drain request-scoped progress notifications already queued when the final POST-SSE response arrives.
• Preserve backend ordering by emitting queued progress frames before the final response.
• Add deterministic regression coverage for ordering, empty/nil routes, and SSE write failures.

Testing

• go test -count=1 ./pkg/transport/proxy/streamable
• go vet ./pkg/transport/proxy/streamable

Fixes #6349

[jhrozek]

Reviewed the fix — closes a genuine select-ordering race where a buffered progress notification could be skipped in favor of the final response. Drain-before-final logic is correct given dispatchResponses' strict per-channel ordering, and the new tests cover the ordering, no-op, and write-failure cases. LGTM.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.24%. Comparing base (7f0d08f) to head (4fd4364).
⚠️ Report is 108 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/transport/proxy/streamable/streamable_proxy.go	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6491      +/-   ##
==========================================
- Coverage   78.25%   78.24%   -0.02%     
==========================================
  Files         770      770              
  Lines       75436    75448      +12     
==========================================
- Hits        59035    59033       -2     
- Misses      16396    16410      +14     
  Partials        5        5              

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