Add thread/resume compatibility shim to code app-server

[cbusillo]

Summary

• add a minimal v2 thread/resume handler to code-rs/app-server
• resume live conversations from rollout history and return a ThreadResumeResponse
• alias resumed historical thread ids to the live conversation ids so existing clients can continue to use the original thread id for addConversationListener and sendUserTurn

Why

Codexia expects thread/resume semantics for Codex-compatible backends. code-rs already had the lower-level resumeConversation path, but it did not expose the v2 thread/resume request shape that Codexia can use directly.

This change intentionally adds a compatibility shim rather than porting the full codex-rs thread manager. The goal is to restore native-looking resume/continue behavior for clients like Codexia with the smallest viable app-server change.

Validation

• cargo check -p code-app-server
• cargo test -p code-app-server thread_resume_response_thread_uses_catalog_metadata
• direct JSON-RPC probe against code-app-server confirming thread/resume, addConversationListener, and sendUserTurn all succeed using the historical thread id
• release build verified through the local code app-server binary on PATH

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: eda8e469b7

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[cbusillo]

Addressed the Codex review feedback in a41cc7b2e6.

Validation:

• ./build-fast.sh
• cargo test -p code-app-server code_message_processor --quiet
• manual code app-server JSON-RPC check through the PATH-linked /opt/homebrew/bin/code symlink, confirming thread/resume now honors explicit path, returns the rollout thread id, and applies modelProvider overrides.