fix: omit empty tools array from provider requests

[LyricalNanoha]

Summary

• vLLM and some OpenAI-compatible APIs reject requests containing an empty tools: [] array with 400 Bad Request
• The model connectivity test sends no tools, causing the test to fail against vLLM servers
• Conditionally include the tools field in request bodies only when tools are present, across all three providers (OpenAI Chat, OpenAI Responses, Anthropic)

Error reproduced

连通性测试失败：provider error: OpenAI Chat 400 Bad Request: {"error":{"message":"`tools` must not be an empty array. Either provide at least one tool or omit the field entirely. (parameter=tools)","type":"BadRequestError","param":"tools","code":400}}

Test plan

• cargo check passes
• All existing connectivity tests pass (cargo test -p cursor-server connectivity)
• Manual verification: configure a vLLM endpoint and run the model connectivity test