Consume tool_call_id in Message proto for FunctionResultContent

[Copilot]

PR #62 added tool_call_id field to the Message proto for associating tool responses with prior invocations. This change ensures the SDK properly sets this field when converting FunctionResultContent to protocol messages.

Changes

• GrokChatClient.cs: Set Message.ToolCallId from FunctionResultContent.CallId when creating tool result messages
• ChatClientTests.cs: Add unit tests verifying ToolCallId mapping behavior

Example

When a FunctionResultContent with a CallId is converted:

var messages = new List<ChatMessage>
{
    new(ChatRole.Assistant, [new FunctionCallContent("call-123", "get_time")]),
    new(ChatRole.Tool, [new FunctionResultContent("call-123", "2024-01-01T00:00:00Z")]),
};

// The resulting Message now includes:
// { Role: RoleTool, ToolCallId: "call-123", Content: [...] }

Original prompt

Ensure we properly consume the new tool_call_id in the Message proto from #62

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