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/routers/pipelines.py

@@ -198,7 +198,7 @@ router = APIRouter()
 @router.get('/list')
 async def get_pipelines_list(request: Request, user=Depends(get_admin_user)):
     responses = await get_all_models_responses(request, user)
-    log.debug(f'get_pipelines_list: get_openai_models_responses returned {responses}')
+    log.debug('get_pipelines_list: get_openai_models_responses returned %s', responses)
 
     urlIdxs = [idx for idx, response in enumerate(responses) if response is not None and 'pipelines' in response]
     base_urls = await Config.get('openai.api_base_urls', [])