fix: handle non-serializable objects in agent canvas SSE and state se… (#14210)

…rialization

Agent components (llm.py, agent_with_tools.py, message.py) store
functools.partial objects as deferred streaming handles in their output
slots. When the canvas state gets serialized for SSE events, Redis
commits, or logging, these partials — plus non-copyable objects like
Langfuse clients — crash json.dumps and deepcopy.

Changes:
- canvas_app.py: add default=str to json.dumps for SSE event
serialization (lines 238, 296)
- canvas.py: wrap deepcopy calls in try/except to handle non-copyable
objects (Langfuse clients, etc.), add default=str to final json.dumps
- base.py: add default=str to ComponentParamBase.__str__ to handle
non-serializable objects in component parameters

Closes #14229

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: yzc <yuzhichang@gmail.com>

agent/component/base.py

@@ -96,7 +96,12 @@ class ComponentParamBase(ABC):
         return {name: True for name in self.get_feeded_deprecated_params()}
 
     def __str__(self):
-        return json.dumps(self.as_dict(), ensure_ascii=False)
+        def _serialize_default(obj):
+            if callable(obj):
+                return None
+            logging.warning("ComponentParamBase.__str__: JSON fallback via str() for type=%s", type(obj).__name__)
+            return str(obj)
+        return json.dumps(self.as_dict(), ensure_ascii=False, default=_serialize_default)
 
     def as_dict(self):
         def _recursive_convert_obj_to_dict(obj):