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/images/comfyui.py

@@ -17,7 +17,7 @@ default_headers = {'User-Agent': 'Mozilla/5.0'}
 async def queue_prompt(prompt, client_id, base_url, api_key):
     log.info('queue_prompt')
     p = {'prompt': prompt, 'client_id': client_id}
-    log.debug(f'queue_prompt data: {p}')
+    log.debug('queue_prompt data: %s', p)
     try:
         session = await get_session()
         async with session.post(
@@ -202,7 +202,7 @@ async def comfyui_create_image(model: str, payload: ComfyUICreateImageForm, clie
         ) as ws:
             log.info('WebSocket connection established.')
             log.info('Sending workflow to WebSocket server.')
-            log.debug(f'Workflow: {workflow}')
+            log.debug('Workflow: %s', workflow)
             images = await _ws_get_images(ws, workflow, client_id, base_url, api_key)
     except aiohttp.WSServerHandshakeError as e:
         log.exception(f'Failed to connect to WebSocket server: {e}')
@@ -243,7 +243,7 @@ async def comfyui_edit_image(model: str, payload: ComfyUIEditImageForm, client_i
         ) as ws:
             log.info('WebSocket connection established.')
             log.info('Sending workflow to WebSocket server.')
-            log.debug(f'Workflow: {workflow}')
+            log.debug('Workflow: %s', workflow)
             images = await _ws_get_images(ws, workflow, client_id, base_url, api_key)
     except aiohttp.WSServerHandshakeError as e:
         log.exception(f'Failed to connect to WebSocket server: {e}')