docs: add guide on custom model clients

[NeuZhou]

Addresses #6641

Summary

Adds a comprehensive cookbook guide for creating custom model clients in AutoGen. The guide covers:

• When to build a custom client — use cases like proprietary APIs, local models, custom gateways, and testing
• The ChatCompletionClient interface — complete overview of all methods to implement
• Step-by-step implementation — building a client from scratch with full code examples:
  • Class setup and initialization
  • Message format conversion
  • \create()\ and \create_stream()\ implementation
  • Token counting and usage tracking
  • Model info and capabilities
• Mock client for testing — a complete deterministic client useful for unit testing agent workflows
• Adding function/tool calling support — how to handle tool calls with \FunctionCall\ objects
• Tips and best practices — practical advice for production-quality clients

The guide is placed in the Core user guide cookbook (\python/docs/src/user-guide/core-user-guide/cookbook/custom-model-clients.md) and added to the cookbook index.

Why this guide?

As noted in #6641, users frequently need to integrate custom model providers but lack documentation on how to implement the \ChatCompletionClient\ protocol. This guide fills that gap with practical, working examples.

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