fix: improve Claude message conversion

[bbbugg]

修改

• 修复 Claude messages[].role === "system" 转换到 systemInstruction
• 修复 tool_result.content 为 JSON 数组时的 Gemini functionResponse.response 格式
• 修复 tool_result 后跟普通文本时被拆成连续 user 的问题
• fix [Bug] Claude /v1/messages 转换后出现连续 user 角色，导致 Gemini API 返回 400 INVALID_ARGUMENT #200

[Copilot]

Pull request overview

This PR updates the Claude→Gemini request conversion in FormatConverter.translateClaudeToGoogle() to better align with Gemini’s contents/systemInstruction expectations and to fix invalid payloads observed with Claude Code tool-calling conversations (Issue #200).

Changes:

• Aggregate messages[].role === "system" content (in addition to top-level claudeBody.system) into Gemini systemInstruction, and skip those messages during contents conversion.
• Normalize Claude tool_result.content so Gemini functionResponse.response is always an object (including when the source is a JSON array).
• Avoid splitting tool_result + same-message text/image into separate user entries by buffering non-tool_result blocks into pendingToolParts.

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[Copilot]

Pull request overview

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[github-actions]

🎉 此 PR 的修改已在版本 v1.3.2 中发布。
🎉 The changes in this PR have been released in version v1.3.2.