fix: align CodeBuddy upstream compatibility

[orangeboyChen]

Summary

• Align Chat and Responses upstream headers with CodeBuddy CLI 2.137.1.
• Adapt Responses upstream requests for Anthropic messages and configurable first system prompt normalization.
• Preserve access keys when all bound credentials disappear, including support for empty credential bindings.

Validation

• bun run lint
• bun run format:check
• bun run typecheck
• bun run test:coverage (93.69% statements, 94.27% lines)
• bun run test:ci
• bun run test:patch-branches --base c943e23e2b623535738732900d5a2f04b5d578c6 (100%)
• bun run build

---

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (c943e23) to head (72704ee).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   94.54%   94.63%   +0.08%     
==========================================
  Files          20       20              
  Lines        4090     4097       +7     
  Branches     1184     1182       -2     
==========================================
+ Hits         3867     3877      +10     
+ Misses        223      220       -3     

Flag	Coverage Δ
unittests	94.63% <100.00%> (+0.08%)	⬆️

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