feat(ai-pipeline): 支持 reasoning signature 及媒体内容块的跨格式流式转换

- 在 CanonicalStreamEvent 中新增 ReasoningSignature 和 ContentPart 变体，CanonicalContentPart 枚举覆盖图片/文件/音频
- Gemini 流解析器提取 thoughtSignature，并对 inlineData/fileData 等非文本 part 生成 ContentPart 事件
- Claude 流聚合支持 thinking_delta / signature_delta，输出 thinking block 携带 signature 字段
- Gemini 同步响应聚合重写，支持媒体 part 和 reasoning signature 的完整还原
- 跨格式矩阵（gemini↔claude↔openai）补全图片块双向转换及 reasoning signature 传递
- 请求转换层（to/from openai_chat）补全 Claude/Gemini 的 thinking、图片、工具调用字段映射
- 新增/扩展测试：inline image 双向重写、thinking signature 聚合、跨格式 sync product 媒体字段

apps/aether-gateway/src/tests/ai_execute/finalize_local_provider/gemini.rs

@@ -912,6 +912,7 @@ async fn gateway_executes_gemini_chat_sync_upstream_stream_via_local_finalize_re
     assert_eq!(
         response_json,
         json!({
+            "responseId": "resp-local-stream",
             "candidates": [{
                 "content": {
                     "parts": [{"text": "Hello Gemini"}],
@@ -1972,10 +1973,18 @@ async fn gateway_executes_antigravity_gemini_cli_sync_upstream_stream_via_local_
     assert_eq!(
         response_json,
         json!({
+            "responseId": "resp-local-stream",
             "_v1internal_response_id": "resp_antigravity_cli_sync_123",
             "candidates": [{
                 "content": {
                     "parts": [
+                        {
+                            "functionCall": {
+                                "name": "get_weather",
+                                "args": {"city": "SF"},
+                                "id": "call_get_weather_0"
+                            }
+                        },
                         {"text": "Hello Antigravity CLI"},
                         {
                             "functionCall": {