fix(xai): cast empty tool properties to object in xAI ToolMap

[Cbrad24]

Problem

When a tool has no parameters/inputs, the xAI provider sends
"properties": [] (JSON array)
instead of
"properties": {} (JSON object)
in the tool definition.

xAI's API rejects this with a 400 error.

This is the same class of PHP empty-array serialization issue fixed in #896 for MessageMap, but in ToolMap (tool definitions rather than tool call arguments).

Before

{
    "type": "function",
    "function": {
        "name": "list_users",
        "description": "List all users",
        "parameters": {
            "type": "object",
            "properties": [],  // Causes 400 error
            "required": []
        }
    }
}

After

{
    "type": "function",
    "function": {
        "name": "list_users",
        "description": "List all users",
        "parameters": {
            "type": "object",
            "properties": {},  // Works
            "required": []
        }
    }
}

Fix

Cast $tool->parametersAsArray() to (object) so empty arrays serialize as {}.
When tool parameters exist, the cast produces an identical JSON object, no behavioral changes.