[codex] Add owner nudge app-server API

[richardopenai]

Summary

Second PR in the split from #17956. Stacked on #18227.

• adds app-server v2 protocol/schema support for account/sendAddCreditsNudgeEmail
• adds the backend-client send_add_credits_nudge_email request and request body mapping
• handles the app-server request with auth checks, backend call, and cooldown mapping
• adds the disabled workspace_owner_usage_nudge feature flag and focused app-server/backend tests

Validation

• cargo test -p codex-backend-client
• cargo test -p codex-app-server-protocol
• cargo test -p codex-app-server rate_limits
• cargo test -p codex-tui workspace_
• cargo test -p codex-tui status_
• just fmt
• just fix -p codex-backend-client
• just fix -p codex-app-server-protocol
• just fix -p codex-app-server
• just fix -p codex-tui

[aibrahim-oai]

let's use the feature flag

[aibrahim-oai]

it's used in the 3rd one

[chatgpt-codex-connector]

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