fix(config): unify log_level from config.yaml across Gateway and debug entry points (#2601)

Centralize log level parsing in `logging_level_from_config()` and
application in `apply_logging_level()` within `deerflow.config.app_config`.

- Gateway lifespan applies configured log level on startup
- `debug.py` uses shared helpers instead of local duplicates
- `apply_logging_level()` targets only `deerflow`/`app` logger hierarchies
  so third-party library verbosity is not affected; root handler levels
  are only lowered (never raised) to allow configured loggers through
  without suppressing third-party output; root logger level is not modified
- Config field description updated to clarify scope
- Tests save/restore global logging state to avoid test pollution

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

backend/debug.py

@@ -34,50 +34,42 @@ _LOG_FMT = "%(asctime)s - %(name)s - %(levelname)s - %(message)s"
 _LOG_DATEFMT = "%Y-%m-%d %H:%M:%S"
 
 
-def _logging_level_from_config(name: str) -> int:
-    """Map ``config.yaml`` ``log_level`` string to a ``logging`` level constant."""
-    mapping = logging.getLevelNamesMapping()
-    return mapping.get((name or "info").strip().upper(), logging.INFO)
+def _setup_logging(log_level: int = logging.INFO) -> None:
+    """Route logs to ``debug.log`` using *log_level* for the initial root/file setup.
 
+    This configures the root logger and the ``debug.log`` file handler so logs do
+    not print on the interactive console. It is idempotent: any pre-existing
+    handlers on the root logger (e.g. installed by ``logging.basicConfig`` in
+    transitively imported modules) are removed so the debug session output only
+    lands in ``debug.log``.
 
-def _setup_logging(log_level: str) -> None:
-    """Send application logs to ``debug.log`` at *log_level*; do not print them on the console.
-
-    Idempotent: any pre-existing handlers on the root logger (e.g. installed by
-    ``logging.basicConfig`` in transitively imported modules) are removed so the
-    debug session output only lands in ``debug.log``.
+    Note: later config-driven logging adjustments may change named logger
+    verbosity without raising the root logger or file-handler thresholds set
+    here, so the eventual contents of ``debug.log`` may not be filtered solely by
+    this function's ``log_level`` argument.
     """
-    level = _logging_level_from_config(log_level)
     root = logging.root
     for h in list(root.handlers):
         root.removeHandler(h)
         h.close()
-    root.setLevel(level)
+    root.setLevel(log_level)
 
     file_handler = logging.FileHandler("debug.log", mode="a", encoding="utf-8")
-    file_handler.setLevel(level)
+    file_handler.setLevel(log_level)
     file_handler.setFormatter(logging.Formatter(_LOG_FMT, datefmt=_LOG_DATEFMT))
     root.addHandler(file_handler)
 
 
-def _update_logging_level(log_level: str) -> None:
-    """Update the root logger and existing handlers to *log_level*."""
-    level = _logging_level_from_config(log_level)
-    root = logging.root
-    root.setLevel(level)
-    for handler in root.handlers:
-        handler.setLevel(level)
-
-
 async def main():
     # Install file logging first so warnings emitted while loading config do not
     # leak onto the interactive terminal via Python's lastResort handler.
-    _setup_logging("info")
+    _setup_logging()
 
     from deerflow.config import get_app_config
+    from deerflow.config.app_config import apply_logging_level
 
     app_config = get_app_config()
-    _update_logging_level(app_config.log_level)
+    apply_logging_level(app_config.log_level)
 
     # Delay the rest of the deerflow imports until *after* logging is installed
     # so that any import-time side effects (e.g. deerflow.agents starts a