Detect the Gemini API error 'function call turn comes immediately after
a user turn or after a function response turn' which indicates corrupted
session history.
When detected:
- Delete the corrupted transcript file
- Remove the session entry from the store
- Return a user-friendly message asking them to retry
This prevents the error loop where every subsequent message fails with
the same error until manual intervention.
Fixes#296
eadb923000