[backlog] Codex OAuth gpt-5.5 support (upstream #571)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #571
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: unknown-needs-runtime · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Codex OAuth provider: `service/oauth/codex.go`; model discovery status types `service/oauth/account.go`; quota path references `gpt-5.4` in `service/oauth/quota.go`. No hard block of gpt-5.5 found statically; needs runtime OAuth+model probe.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Runtime probe confirms whether gpt-5.5 is blocked by discovery, quota, or routing

• Allowed model list includes gpt-5.5 when provider supports it

• OAuth account can complete a gpt-5.5 request end-to-end

• Tests/fixtures updated for model allowlist without hardcoding obsolete models only

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.