* fix: codex free accounts incorrectly marked as quota exhausted when skip_exhausted_accounts enabled
- Cross-validate stored exhausted flag against window used_ratio in snapshot reader
- Guard has_credits:false check with window data presence in upstream_metadata fallback
- Add windows.is_empty() precondition to exhausted_by_credits in snapshot builder
Free codex accounts have has_credits:false (they use window-based quotas, not credits),
but the old logic treated this as credits depleted, skipping actual window usage checks.
This caused all free accounts to be incorrectly marked exhausted.
* test: cover codex free quota windows
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Co-authored-by: root <[email protected]>
Co-authored-by: fawney19 <[email protected]>