docs(serve): say why turn_in_flight is kept as the bool view; one union spelling

hermes_cli/web_server_idle_exit.py

@@ -137,7 +137,8 @@ def busy_ledger() -> Optional[str]:
 
 
 def turn_in_flight() -> Optional[bool]:
-    """True/False from :func:`busy_ledger`; None when it cannot be read."""
+    """Bool view of :func:`busy_ledger`, kept so ``should_exit_idle`` / the idle watchdog and injected
+    test probes keep their ``Optional[bool]`` contract; None when the ledgers cannot be read."""
     ledger = busy_ledger()
     return None if ledger is None else bool(ledger)
 

hermes_cli/web_server_idle_proof.py

@@ -36,7 +36,7 @@ def pending_human_input() -> Optional[int]:
         return None
 
 
-def idle_proof(turn_probe: Callable[[], Optional[bool | str]] = busy_ledger,
+def idle_proof(turn_probe: Callable[[], bool | str | None] = busy_ledger,
                input_probe: Callable[[], Optional[int]] = pending_human_input) -> dict:
     """``{"idle": True | False | None, "reason": str | None}`` plus ``"detail"`` naming the busy ledger.