feat: Support xAI native client and openai compatible response client

[Wendong-Fan]

Summary

Add native xAI model backend using the xai_sdk gRPC package, enabling direct access to Grok models (including grok-4.20-reasoning)
with native features that are unavailable through the OpenAI-compatible endpoint.

Key features

• Native xAI SDK integration: Uses xai_sdk (gRPC) instead of the OpenAI-compatible REST endpoint, providing access to xAI-specific
  features like encrypted thinking content and server-side conversation storage.
• Conversation chaining via previous_response_id: On multi-turn conversations, only delta messages are sent to the API. The server
  retains the full history and applies automatic prompt caching, significantly reducing token usage and cost.
• Encrypted thinking content: Supports use_encrypted_content for reasoning models. Encrypted reasoning traces are stored across turns
  and injected back into the conversation history, preserving the full reasoning chain.
• Full message format conversion: Translates between CAMEL's OpenAI-format messages and xAI SDK protobuf messages
  (system/user/assistant/tool roles, including tool_calls and tool results).
• Streaming support: Converts xAI's (Response, Chunk) tuple stream into OpenAI-compatible ChatCompletionChunk generators with proper
  pydantic objects.
• Sync and async: Both _run() and _arun() are supported via lazy-initialized xai_sdk.Client and xai_sdk.AsyncClient.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch xai_native

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