fix: handle partial missing usage in platform fallback

[kgskr]

Summary

• treat a ChatGPT candidate as healthy for Platform fallback suppression only when both required usage snapshots are present and healthy
• prevent all-missing and partial-missing usage snapshots from keeping Platform fallback idle
• sync OpenSpec wording and add regression coverage for public and backend fallback paths

Problem

The previous fallback logic still allowed a mixed pool case to block Platform fallback:

• one ChatGPT account already drained
• another compatible ChatGPT account missing one of the persisted usage windows

That partial-missing candidate could still keep the request on the ChatGPT path even though it was not positively healthy.

Changes

• tighten app/modules/proxy/load_balancer.py fallback health evaluation to require both known windows
• add unit coverage for:
  • all-missing usage
  • partial-missing usage
  • weekly-only usage
• add integration coverage for:
  • public /v1/responses mixed-pool partial-missing fallback
  • backend Codex HTTP /backend-api/codex/responses mixed-pool partial-missing fallback
• update OpenSpec to state that either missing window disqualifies a ChatGPT candidate from suppressing Platform fallback
• update existing healthy-primary tests so they seed both windows and represent a truly healthy candidate

Validation

• ruff check app/modules/proxy/load_balancer.py tests/unit/test_load_balancer.py tests/unit/test_public_route_selection.py tests/integration/test_openai_platform_proxy.py
• pytest tests/unit/test_load_balancer.py -q
• pytest tests/unit/test_public_route_selection.py -q
• pytest tests/integration/test_openai_platform_proxy.py -q
• openspec validate --specs

[kgskr]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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