app-server: treat null mode developer instructions as built-in defaults

[charley-oai]

Summary

• make turn/start normalize collaborationMode.settings.developer_instructions: null to the built-in instructions for the selected mode
• prevent app-server clients from accidentally clearing mode-switch developer instructions by sending null
• document this behavior in the v2 protocol and app-server docs

What changed

• codex-rs/app-server/src/codex_message_processor.rs
  • added a small normalize_turn_start_collaboration_mode helper
  • in turn_start, apply normalization before OverrideTurnContext
• codex-rs/app-server/tests/suite/v2/turn_start.rs
  • extended turn_start_accepts_collaboration_mode_override_v2 to assert the outgoing request includes default-mode instruction text when the client sends developer_instructions: null
• codex-rs/app-server-protocol/src/protocol/v2.rs
  • clarified TurnStartParams.collaboration_mode docs: settings.developer_instructions: null means use built-in mode instructions
• regenerated schema fixture:
  • codex-rs/app-server-protocol/schema/typescript/v2/TurnStartParams.ts
• docs:
  • codex-rs/app-server/README.md
  • codex-rs/docs/codex_mcp_interface.md

[charley-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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