fix: map Chat usage onto Responses usage counters

[orangeboyChen]

Problem

/v1/responses returned token counts that clients could not read:

• Non-streamed: the upstream Chat usage object was forwarded verbatim, so callers received prompt_tokens / completion_tokens where the Responses API contract requires input_tokens / output_tokens.
• Streamed: response.completed emitted no usage at all.

Codex and other Responses-style clients read the input_tokens / output_tokens fields, so they displayed zeroed or missing token counts even though upstream reported real values.

Fix

Added a shared mapChatUsageToResponses helper in lib/server/proxy/responses.ts and applied it to both paths:

• prompt_tokens → input_tokens, completion_tokens → output_tokens
• cached prompt tokens → input_tokens_details.cached_tokens (reads prompt_tokens_details.cached_tokens, input_tokens_details.cached_tokens, cache_read_input_tokens, or prompt_cache_hit_tokens)
• reasoning tokens → output_tokens_details.reasoning_tokens
• total_tokens preserved, with a computed fallback when upstream omits it
• zeroed counters when upstream omits usage entirely

Streaming now captures the aggregated usage from the final upstream Chat chunk and includes it in the downstream response.completed event.

Verification

• bun run lint
• bun run format:check
• bun run typecheck
• bun run test:coverage — 247 tests passing, 94.01% statements (threshold 90%)
• bun run build

Added three regression tests in tests/server/responses-memory.test.ts covering non-streamed mapping, streamed response.completed usage, and the missing-usage fallback.

[chatgpt-codex-connector]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.83%. Comparing base (7845b54) to head (8330d0a).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   94.77%   94.83%   +0.05%     
==========================================
  Files          21       21              
  Lines        4251     4300      +49     
  Branches     1238     1257      +19     
==========================================
+ Hits         4029     4078      +49     
  Misses        222      222              

Flag	Coverage Δ
unittests	94.83% <100.00%> (+0.05%)	⬆️

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