feat(codex): add exact account routing

[chrisae9]

Part of #425. This is the second layer split out of #426 after the maintainer review; the namespace foundation is already on dev as c9bed7c5a.

What this adds

An account-qualified native model now routes to the one stored Codex account mapped by its namespace. For example, side/gpt-5.6-sol uses the side binding and sends gpt-5.6-sol on the wire through the canonical openai provider.

• Exact selection works even when the OpenAI provider is globally in Direct mode.
• Bare gpt-* models keep their existing Direct or Pool behavior.
• Missing, unusable, paused, cooling-down, or reauthentication-required accounts fail closed.
• Exact requests do not run Pool balancing, quota priming/probes, affinity rebinding, active-account promotion, alternate-account retry, or subagent fallback.
• Physical account health is still recorded, but an exact request cannot rotate or otherwise mutate Pool selection.
• Same-credential transient transport retry remains available.
• Logs and client-facing cooldown errors use the public namespace, never the stored account ID.

The binding is carried through Responses HTTP/SSE, compact, WebSocket response.create, and the translated chat/Claude paths that use the shared Responses core. Standalone endpoints that do not receive the qualified model selector (Images, Search, and Live) retain normal provider routing.

The latest dev pause controls are respected: pausing an account blocks new exact selections without invalidating an already-started request.

Not in this PR

Catalog generation/model-picker rows, account lifecycle and settings UI, documentation/localization, and account-qualified fallback-chain candidates remain separate follow-up layers. Keeping those out is intentional so this review stays focused on the credential-routing boundary.

Security review

This changes credential-to-model routing and therefore needs the explicit maintainer security review required by MAINTAINERS.md.

The main invariants are covered by regressions for Direct override, exact main and added accounts, missing/paused/cooled accounts, 400/402/429 handling, HTTP/SSE, compact, WebSocket, public-only logging, quota probes, Pool state preservation, and spawned-child fallback behavior.

Validation

• bun run prepush
• Typecheck passed
• GUI lint passed
• 5,897 tests passed, 1 platform-specific test skipped, 0 failed
• Privacy scan passed
• React Doctor skipped because this PR does not change gui/

Per the current integration policy, this TypeScript routing change will also need its dev2-go counterpart (or a recorded no-counterpart decision) after it lands on dev.

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