feat: support OpenAI tools call

[bbbugg]

• 支持Openai 接口的工具调用
• 支持Gemini 3系列的模型工具调用，补全thoughtSignature
• 启用日志等级，默认INFO，可以通过UI和环境变量设为DEBUG

相关文档：https://ai.google.dev/gemini-api/docs/thought-signatures?hl=zh-cn

[Copilot]

Pull request overview

This PR adds comprehensive support for OpenAI tools/function calling by implementing bidirectional conversion between OpenAI and Gemini API formats. The implementation handles tool call requests, tool responses, and ensures compatibility with Gemini's thoughtSignature requirements.

• Converts OpenAI tool_calls and tool role messages to Gemini functionCall/functionResponse format
• Converts Gemini functionCall responses back to OpenAI tool_calls format for both streaming and non-streaming responses
• Adds thoughtSignature placeholder handling to bypass Gemini API validation requirements

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 25 comments.

File	Description
src/core/FormatConverter.js	Implements core translation logic for OpenAI tools ↔ Gemini function calling, including tool definition conversion, message buffering for consecutive tool messages, and bidirectional response translation for both streaming and non-streaming modes
src/core/RequestHandler.js	Integrates thoughtSignature validation bypass by calling ensureThoughtSignature for native Google requests with function calls/responses

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 14 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.