Add workspace announcement polling client

[xli-oai]

Summary

• Stack on Add workspace message backend client plumbing #21223 for shared backend-client models, read method, and feature flag.
• Start an app-server background poller for announcement workspace messages only when workspace_messages is enabled.
• Poll every 15 minutes with a 5s timeout, fail open on auth/network/timeouts, and fetch the actual announcement messages while preserving backend order.
• Do not ack or render yet.

Validation

• just fmt
• cargo test --manifest-path codex-rs/Cargo.toml -p codex-backend-client
• cargo check --manifest-path codex-rs/Cargo.toml -p codex-app-server
• cargo test --manifest-path codex-rs/Cargo.toml -p codex-features

[chatgpt-codex-connector]

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Reviewed commit: e04aa0c12f

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[xli-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".