feat(openai-codex): persist and send Fast priority mode

[WebMad]

Responsibility and scope

Adds backend/configuration support for persisting the OpenAI Codex speed preference and mapping Fast mode to the provider request priority field. Tests travel with the behavior in this PR.

Behavior

• Fast sends Priority in the OpenAI Codex request.
• Missing and Default settings omit the priority field.
• Flex is rejected for this provider path.
• Persistence, validation, serialization, and request construction are covered at the backend layer.

Explicit non-goals

• No webview-ui/ changes and no user-facing selector; that is the next stack PR.
• No unrelated Task.throttle.test.ts fix.
• No changeset.

Test evidence

• Focused backend tests cover Fast, missing/Default omission, and Flex rejection.
• Repository type checks passed from the push hook.
• The completed stack was locally verified before publication.

Stack dependency

1. Zoo-Code-Org/Zoo-Code#1040 — shared service-tier primitives.
2. This PR — backend persistence and request behavior (base: WebMad:refactor/service-tier-primitives).
3. WebMad/Zoo-Code#3 — webview speed selector and translations.

Review after Zoo-Code-Org#1040; this PR intentionally contains only the immediate-base backend delta.