[codex] Sanitize imported session fallback titles

[stefanstokic-oai]

Summary

• sanitize Claude-derived fallback titles without changing explicit custom or generated source titles
• recognize known leading control wrappers, including ide_opened_file, ide_selection, local-command-stdout, and local-command-stderr
• skip separate control-only user records and use the next meaningful user message as the fallback title
• use Imported session only when every user record is control-only
• preserve raw imported messages, previews, and transcript provenance unchanged

Why

Sessions without an explicit source title fall back to user-message content. Claude metadata can arrive as separate leading user records, so stripping only the first record still exposed raw control markup or produced an unhelpful fallback.

Impact

Imported chats now receive readable fallback titles such as Fix auth flow, while the original wrapper-bearing messages remain visible in the imported transcript.

Validation

• just test -p codex-external-agent-sessions — 37 tests passed
• just test -p codex-app-server external_agent_config_import_creates_session_rollouts — focused integration test passed
• just fix -p codex-external-agent-sessions
• just fix -p codex-app-server
• just bazel-lock-check
• just fmt