## What changed
Remove `initial_messages` from `SessionConfiguredEvent` and the corresponding schema. Stop attaching replay history to startup events for resumed or rejoined threads, and remove `InitialHistory::get_event_msgs`.
## Testing
Update the MCP resume test to verify persisted tool events through `load_history`, and retain coverage that resumed requests include prior conversation items.
GitOrigin-RevId: 54b72bc44423c9b1ef2051891ef392c0ede32fe5
## Why
`CodexThread` retained the full `SessionConfiguredEvent`, including initial replay messages, and cloned it even for callers that only needed thread or session IDs.
## What changed
Store `ThreadStartupMetadata` without replay history and expose it by reference through `startup_metadata()`. Update callers to read IDs from this metadata. When rejoining a running thread, rebuild the startup response using the supplied resume history for `initial_messages`.
## Testing
Extend the resume test to verify that rejoining reuses the loaded thread, preserves startup metadata, and still returns the original replay messages as a prefix.
GitOrigin-RevId: 9d656063088c28f091d29e5862e4b9d894b7642c
Owen Lin