perf: build debug log messages lazily so disabled debug logs cost nothing (#27834)

GLOBAL_LOG_LEVEL defaults to INFO, so every log.debug(...) in the backend is discarded, but the message is built first: 187 call sites interpolate their payload into an f-string before the logging call runs, so the work happens on every request and the result is thrown away. The worst one sits in process_chat_payload and stringifies the whole request body, full conversation history included, once per chat completion.

That one line with DEBUG disabled, CPython 3.12:

| conversation | payload | before   | after   |
| ------------ | ------- | -------- | ------- |
| 4 messages   | 1.2 kB  | 3.4 us   | 0.07 us |
| 20 messages  | 17 kB   | 24.8 us  | 0.07 us |
| 60 messages  | 123 kB  | 216.6 us | 0.07 us |

The lazy form log.debug('form_data: %s', form_data) hands the payload to record.getMessage(), which the InterceptHandler only reaches once a record has passed the level check. With DEBUG enabled the emitted lines are byte-identical, f'{x=}' sites included: those map to %r. MistralLoader._debug_log callers get the same treatment, since that wrapper already forwards *args.

backend/open_webui/utils/models.py

@@ -307,7 +307,7 @@ async def get_all_models(request, refresh: bool = False, user: UserModel = None)
         try:
             await get_function_module_from_cache(request, function_id, function=function)
         except Exception as e:
-            log.debug(f'Failed to load function module for {function_id}: {e}')
+            log.debug('Failed to load function module for %s: %s', function_id, e)
 
     # Apply global model defaults to all models
     # Per-model overrides take precedence over global defaults
@@ -413,7 +413,7 @@ async def get_all_models(request, refresh: bool = False, user: UserModel = None)
                 filter_items_by_id[filter_id] = items
             model['filters'].extend({**item} for item in items)
 
-    log.debug(f'get_all_models() returned {len(models)} models')
+    log.debug('get_all_models() returned %s models', len(models))
 
     models_dict = {model['id']: model for model in models}
     if isinstance(request.app.state.MODELS, RedisDict):