Prevent back-to-back live starts from leaking stale transcript data

[yujonglee]

• Root cause:
  • The desktop listener flow installed the new transcript persist callback too early during a back-to-back start.
  • Session B’s transcript row and persist handler could be provisioned before session A had fully finished finalizing.
  • If A still had buffered transcript data, A’s final flush/reset could write into B’s transcript target.
  • The listener plugin also swallowed start_session rejection, so the frontend could proceed as though the new session had started.
• What changed:
  • Reject any new live start while another live session is still active or finalizing.
  • Keep the frontend start path aligned with Rust by returning a real error when start_session is rejected.
  • Remove the provisional transcript row when that start attempt fails.
  • Replace raw rejection-string matching with a typed StartSessionError.
• Why this fixes it:
  • The transcript persist callback is no longer handed off to a new session while the previous session is still finalizing.
  • A stale finalize/reset from session A cannot land in session B’s persisted transcript row.
  • Rejected starts no longer leave behind transcript artifacts that could be mistaken for valid session output.
• Validation:
  • Added a listener store test covering rejected start while another session is finalizing.
  • Ran desktop tests and typecheck.
  • Ran Rust cargo check for listener-core and tauri-plugin-listener.
• Follow-ups:
  • Add a user-facing message when a start is rejected because the previous session is still finalizing.
  • Consider queuing a back-to-back start automatically after finalization if we want smoother handoff UX.
  • If we want stronger long-term isolation, consider session-owned transcript runtime state in the desktop store.

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