[Bug]: JSONEditTool rejects explicit null values for set and add

### What happened?

`JSONEditTool` treats an explicit JSON `null` value as if the `value` argument were missing. As a result, both `set` and `add` reject valid tool calls that contain `"value": null`.

Minimal reproduction:

```python
import asyncio
import json
import tempfile
from pathlib import Path

from trae_agent.tools.base import ToolCallArguments
from trae_agent.tools.json_edit_tool import JSONEditTool

async def main():
with tempfile.TemporaryDirectory() as directory:
path = Path(directory) / "data.json"
path.write_text('{"value": 1, "items": []}')
tool = JSONEditTool()

set_result = await tool.execute(
ToolCallArguments(
{
"operation": "set",
"file_path": str(path),
"json_path": "$.value",
"value": None,
}
)
)
add_result = await tool.execute(
ToolCallArguments(
{
"operation": "add",
"file_path": str(path),
"json_path": "$.items[0]",
"value": None,
}
)
)

print(set_result.error)
print(add_result.error)
print(json.loads(path.read_text()))

asyncio.run(main())
```

Observed output:

```text
A 'value' parameter is required for the 'set' operation.
A 'value' parameter is required for the 'add' operation.
{'value': 1, 'items': []}
```

The check currently uses `value is None`, which cannot distinguish an omitted argument from an explicitly supplied JSON `null`.

### What did you expect to happen?

When the tool call contains the `value` key, `None` should be treated as the JSON value `null`. The example above should produce:

```json
{"value": null, "items": [null]}
```

Only a missing `value` key should return the required-parameter error.

### Traceback

No exception is raised. The tool returns `error_code=-1` with the required-parameter message shown above.

### What is your system, Python, dependency version?

- OS: macOS arm64
- Python: 3.12.14
- trae-agent commit: `e839e55`

### Additional information that you believe is relevant to this bug

In OpenAI strict mode, optional tool parameters are included in `required` and made nullable by `Tool.get_input_schema()`. The generated schema can therefore permit an explicit `null` value while `JSONEditTool.execute()` rejects it at runtime.

The intended fix is to check whether `"value"` is present in the arguments rather than checking whether its value is `None`. Regression tests should cover setting a field to `null`, adding `null` to an array, and continuing to reject an omitted `value` argument.

Start in trae_agent/tools/json_edit_tool.py and inspect JSONEditTool.execute(), using ToolCallArguments from trae_agent/tools/base.py to understand how argument presence is represented. Add regression coverage for setting and adding null values while preserving the missing-value error, then run the relevant JSONEditTool test suite and verify the reproduced JSON output.

Written by the indexing model from the issue text.