fix(triple): allow unary handlers to set response metadata

[Alanxtl]

What changed

• Store the unary handler connection in the handler context before invoking user logic.
• Add a regression test that calls triple.SetHeader and triple.SetTrailer from a unary handler and verifies the client receives both values.

Why

SetHeader and SetTrailer require handlerOutgoingKey{} in the handler context to access the current StreamingHandlerConn. The unary handler path only injected incoming metadata, so generated unary service implementations could not use the documented response metadata APIs.

Closes #3444.

Validation

go test ./protocol/triple/triple_protocol -run TestSetHeaderAndSetTrailerInUnaryHandler -count=1
go test ./protocol/triple/triple_protocol
go test ./protocol/triple

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.65%. Comparing base (60d1c2a) to head (9d4daaa).
⚠️ Report is 852 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3446      +/-   ##
===========================================
+ Coverage    46.76%   53.65%   +6.88%     
===========================================
  Files          295      493     +198     
  Lines        17172    38421   +21249     
===========================================
+ Hits          8031    20613   +12582     
- Misses        8287    16147    +7860     
- Partials       854     1661     +807     

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