Keep settings operations bound to their committed snapshot (#40645)

## Why

Post-commit work can overlap with another settings write. Reading the session
again afterward could make a settings operation report or use the later state
instead of the state that it committed.

## What changed

- Return the committed configuration and `ThreadSettingsSnapshot` from session
  settings updates.
- Use that commit result when creating a turn and emitting
  `ThreadSettingsApplied`, while retaining current-state snapshots for
  synthesized fork history.
- Route turn-start updates through the shared settings update path.

## Testing

Add coverage for standalone and turn-start updates that pause after committing,
restore different settings concurrently, and verify that each operation keeps
its original configuration and notification snapshot.

GitOrigin-RevId: cee5be9536d2840170893e6eb0c64cd039fb0a96