fix: Sub-agents settings show blank and switched off when a limit is 0 (#33612)

Saving the Sub-agents admin tab could turn sub-agents off and clear their system prompt when one of the limits was still 0. Older releases accepted 0 there, also from the SUBAGENTS_MAX_* environment variables, but the tab now fails to load such a value and shows everything blank and switched off, so saving writes that blank state back. A limit of 0 now shows as empty, meaning no limit, which matches what sub-agents already do with a 0 while they run.

Fixes #33609

backend/open_webui/routers/configs.py

@@ -814,7 +814,7 @@ class SubagentsConfigForm(BaseModel):
     )
     @classmethod
     def normalize_limit(cls, value):
-        return None if value == '' else value
+        return None if value in ('', 0) else value
 
     @field_validator('SUBAGENTS_MAX_CONCURRENT', 'SUBAGENTS_MAX_ASYNC', mode='before')
     @classmethod