[tool] fix: avoid nested ToolResponse in SandboxFusionTool (#4833)

### What does this PR do?

Fix an incorrect double-wrapping of `ToolResponse` in
`SandboxFusionTool.execute()`.

- `execute_code()` already returns a `ToolResponse`, but `execute()`
previously wrapped it again as `ToolResponse(text=result)`.
- Since `ToolResponse.text` expects `str | None`, the old behavior could
produce an invalid/nested response (or confusing stringified output).
- This PR makes `execute()` return the `ToolResponse` directly when
appropriate, and only wraps once when the worker returns a
non-`ToolResponse` result.

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### API and Usage Example

No API changes. `SandboxFusionTool.execute()` still returns
`tuple[ToolResponse, float, dict]`.

```python
# Add code snippet or script demonstrating how to use this
```

### Design & Code Changes

- `verl/tools/sandbox_fusion_tools.py`
- If the execution worker returns a `ToolResponse`, return it directly.
- Otherwise, convert the result to `str` (or `None`) and wrap once as
`ToolResponse(text=...)`.

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Co-authored-by: winston <email@example.com>

Author: Winston-Yuan

verl/tools/sandbox_fusion_tools.py

@@ -174,7 +174,9 @@ async def execute(self, instance_id: str, parameters: dict[str, Any], **kwargs)
 
         result = await self.execution_pool.execute.remote(self.execute_code, instance_id, code, timeout, language)
         # sandbox has no score or metrics, use Nones
-        return ToolResponse(text=result), None, None
+        if isinstance(result, ToolResponse):
+            return result, None, None
+        return ToolResponse(text=None if result is None else str(result)), None, None
 
     def execute_code(self, instance_id, code, timeout=30, language="python"):
         result_status, metadata = _process_single_case(