fix(codex-pool): skip accounts with zero quota

[huangzesen]

Summary

Implement only the agreed zero-quota selection path:

• internal read_remaining_percent(auth_path) -> float | None uses Codex app-server account/rateLimits/read and reads only the primary window
• a query failure or malformed/non-finite/out-of-range field returns None
• new codex-pool selection excludes only accounts explicitly reporting <= 0; None remains eligible
• surviving weights and original source indices are preserved; existing sessions are untouched
• if every validated account reports zero, raise CodexPoolAllAccountsExhaustedError("All Codex pool accounts are exhausted")

No public quota API, snapshot/reporting surface, cache/TTL, vision special case, adapter API, dashboard metadata, or extra state machine.

Diff

7 paths, +526 / -14 versus base. This replaces the original +2130 / -32 candidate.

Validation

• exactly 6 focused feature tests: positive, zero, unavailable fail-open, weights/index, all-zero, immediate reset re-entry — PASS
• 109 existing test_codex_pool.py regressions — PASS
• combined: 115 PASS
• touched production modules compile; git diff --check PASS
• frozen patch SHA256: fb18a619d1fa4b2ed450bf716e93e8df605945f10a61aab43430c2dba7e895e6
• independent Terra review of the byte-identical snapshot: PASS, no P0/P1