修复 CodeBuddy 不支持非流式请求的问题

[rensumo]

变更说明

• 修复 CodeBuddy 在非流式请求场景下无法正常返回的问题
• 调整 CodeBuddy 执行器的非流式逻辑：上游统一走 SSE 流式请求，再将返回的 chat.completion.chunk 聚合为单次非流式 chat.completion 响应
• 保持原有流式行为不变，同时保留 usage、reasoning_content、tool_calls、finish_reason 等关键信息，确保后续响应转换逻辑可正常工作

[gemini-code-assist]

Code Review

This pull request implements streaming response aggregation for the CodeBuddy executor. Key changes include updating request translation to support streaming, adding required HTTP headers, and introducing the aggregateOpenAIChatCompletionStream function to merge stream chunks into a standard chat completion format. Review feedback identified a critical logic error where the translated payload was being incorrectly overwritten and suggested optimizing redundant JSON parsing during the stream aggregation process.

[gemini-code-assist]

在 aggregateOpenAIChatCompletionStream 的循环内部存在重复的 JSON 解析。gjson.ParseBytes(payload) 已经解析了整个对象，但随后的 parseOpenAIStreamUsage(line) 内部又会再次对同一段数据进行解析。虽然在非流式聚合场景下性能开销通常可接受，但建议优化以减少冗余解析，例如直接从已解析的 root 对象中提取 usage 信息。