FDRYMAF015: Do not call ToString on tool result objects

Cause

Code calls .ToString() on a tool result property such as ToolCallResult.Result or FunctionResultContent.Result.

Why it matters

The value may be a JsonElement. Calling .ToString() can lose the original JSON representation for arrays and complex objects.

How to fix

Use ToolResultSerializer.Serialize() to produce a stable string representation.

var text = ToolResultSerializer.Serialize(toolResult.Result);