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/tools.py

@@ -1187,7 +1187,7 @@ async def get_terminal_cwd(
                     data = await resp.json()
                     return data.get('cwd')
     except Exception as e:
-        log.debug(f'Failed to fetch terminal CWD: {e}')
+        log.debug('Failed to fetch terminal CWD: %s', e)
     return None
 
 
@@ -1226,7 +1226,7 @@ async def get_terminal_system_prompt(
                     data = await resp.json()
                     return data.get('prompt')
     except Exception as e:
-        log.debug(f'Failed to fetch terminal system prompt: {e}')
+        log.debug('Failed to fetch terminal system prompt: %s', e)
     return None
 
 
@@ -1447,7 +1447,7 @@ async def get_tool_server_data(url: str, headers: dict | None) -> dict[str, Any]
             error = str(err)
         raise Exception(error)
 
-    log.debug(f'Fetched data: {res}')
+    log.debug('Fetched data: %s', res)
     return res