[codex-analytics] add session source to client metadata

[marksteinbrick-oai]

Summary

Adds thread_source field to the existing Codex turn metadata sent to Responses API

• Sends thread_source: "user" for user-initiated sessions: CLI, VS Code, and Exec
• Sends thread_source: "subagent" for subagent sessions
• Omits thread_source for MCP, custom, and unknown session sources
• Uses the existing turn metadata transport:
  • HTTP requests send through the x-codex-turn-metadata header
  • WebSocket response.create requests send through client_metadata["x-codex-turn-metadata"]

Testing

• cargo test -p codex-protocol session_source_thread_source_name_classifies_user_and_subagent_sources
• cargo test -p codex-core turn_metadata_state
• cargo test -p codex-core --test responses_headers responses_stream_includes_turn_metadata_header_for_git_workspace_e2e -- --nocapture

[github-actions]

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[marksteinbrick-oai]

I have read the CLA Document and I hereby sign the CLA