[codex] Add workspace owner usage nudge UI

[richardopenai]

Summary

Third PR in the split from #17956. Stacked on #18220.

• shows workspace-owner/member-specific rate-limit messages behind workspace_owner_usage_nudge
• prompts workspace members to notify the owner or request a usage-limit increase
• sends the confirmed nudge through the app-server API and renders completion feedback
• adds focused TUI snapshot coverage for prompts and completion states
• feature gate

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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clear stale reached-type cache when snapshot is unclassified

on_rate_limit_snapshot only updates codex_rate_limit_reached_type when the incoming codex snapshot has a value. When rate_limit_reached_type is None, the previous value is retained, but on_rate_limit_error later uses this cache to pick owner/member-specific messaging and nudge behavior. That can surface the wrong prompt/message after account state changes and a later 429 arrives without classification.

Useful? React with 👍 / 👎.

[richardopenai]

/merge