fix(ai): retry unavailable Codex upstreams

[minpeter]

Summary

• classify transient upstream_unavailable Codex websocket proxy failures as retryable
• preserve the existing bounded retry policy and retry event contract
• replace stale fix(ai): retry upstream availability failures #323 cleanly from current main without force-pushing history

RED -> GREEN

• RED: the reported Error: upstream_unavailable: ... ConnectionClosedOK classified as non-retryable
• GREEN: classifier and full coding-agent retry event-contract suites pass
• real print-mode CLI surface: first local Responses request failed with the exact error, second recovered; exit 0, 2 requests, recovered output, unchanged real auth

Validation

• packages/ai/test/retry.test.ts: 7 passed
• packages/coding-agent/test/suite/retry-fallback-engine.test.ts: 10 passed
• root npm run build: passed
• root npm run check: passed
• git diff --check: passed

Supersedes #323, which was based on pre-#310 main and conflicts only in the rewritten retry fixture.

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Summary by cubic

Classifies Codex upstream_unavailable websocket proxy errors as transient so they retry under the existing bounded policy. Prevents immediate failure on disconnects and allows automatic recovery.

• Bug Fixes
  • Mark "upstream_unavailable" provider errors as retryable in utils/retry.ts.
  • Add tests in packages/ai and packages/coding-agent to cover Codex websocket disconnects.
  • Preserve current retry limits and retry event contract.

^{Written for commit f6c3f65. Summary will update on new commits.}