[codex] fix OpenAI Codex primary 5h quota reset

[anzhiyu-c]

Summary

• Treat primary-window Codex 5h quota headers as used percent instead of inverting them as remaining percent.
• Keep the legacy secondary-window 5h remaining-percent conversion.
• Add regression cases for both an exhausted primary 5h window and an available primary 5h window (primary_used_percent=0).

Root cause

When OpenAI sends the 5h Codex limit in the primary quota window (x-codex-primary-window-minutes: 300), x-codex-primary-used-percent is already a used percentage. The previous normalization always calculated 100 - raw for the canonical 5h used percent, so a primary 5h value of 0 became 100. That can make an account with available 5h quota display as fully exhausted.

The same inversion also affects reset selection when primary=5h is exhausted, because the canonical 5h used percent may no longer reflect the actual upstream limit state.

Validation

• go test -tags=unit ./internal/service -run 'TestCalculateOpenAI429ResetTime|TestOpenAIUpdateCodexUsageSnapshotFromHeaders|TestNormalizedCodexLimits'\n- go test -tags=unit ./internal/service

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