(bugfix) Keep rsync runner callback optional

Only pass the process_started hook when live run state tracking is active,
so existing rsync call sites and tests without that hook remain compatible.

Refs #54

src/pobsync/commands/run_scheduled.py

@@ -366,13 +366,14 @@ def run_scheduled(
                 }
             )
 
-        result = run_rsync(
-            cmd,
-            log_path=log_path,
-            timeout_seconds=timeout_seconds,
-            cancel_check=cancel_check,
-            process_started=process_started if state_callback is not None else None,
-        )
+        run_rsync_kwargs: dict[str, Any] = {
+            "log_path": log_path,
+            "timeout_seconds": timeout_seconds,
+            "cancel_check": cancel_check,
+        }
+        if state_callback is not None:
+            run_rsync_kwargs["process_started"] = process_started
+        result = run_rsync(cmd, **run_rsync_kwargs)
         log_tail = _read_log_tail(log_path)
         warning = classify_rsync_warning(result.exit_code, log_tail)
         successful_or_warning = result.exit_code == 0 or warning is not None