fix: Handle NaN/Infinity values in ExeSQL JSON response (#12666)

## Summary

Fixes #12631

When SQL query results contain NaN (Not a Number) or Infinity values
(e.g., from division by zero or other calculations), the JSON
serialization would fail because **NaN and Infinity are not valid JSON
values**.

This caused the agent interface to show 'undefined' error, as described
in the issue where `EXAMINE_TIMES` became `NaN` and broke the JSON
parsing.

## Root Cause

The `convert_decimals` function in `exesql.py` was only handling
`Decimal` types, but not `float` values that could be `NaN` or
`Infinity`.

When these invalid JSON values were serialized:
```json
{"EXAMINE_TIMES": NaN}  // Invalid JSON!
```

The frontend JSON parser would fail, causing the 'undefined' error.

## Solution

Extended `convert_decimals` to detect `float` values and convert
`NaN`/`Infinity` to `null` before JSON serialization:

```python
if isinstance(obj, float):
    if math.isnan(obj) or math.isinf(obj):
        return None
    return obj
```

This ensures all SQL results can be properly serialized to valid JSON.

---
This is a Gittensor contribution.
gittensor:user:GlobalStar117

Co-authored-by: GlobalStar117 <GlobalStar117@users.noreply.github.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

agent/tools/exesql.py

@@ -86,6 +86,12 @@ class ExeSQL(ToolBase, ABC):
 
         def convert_decimals(obj):
             from decimal import Decimal
+            import math
+            if isinstance(obj, float):
+                # Handle NaN and Infinity which are not valid JSON values
+                if math.isnan(obj) or math.isinf(obj):
+                    return None
+                return obj
             if isinstance(obj, Decimal):
                 return float(obj)  # 或 str(obj)
             elif isinstance(obj, dict):