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_cli/cross_format.rs

@@ -860,6 +860,7 @@ async fn gateway_executes_openai_cli_cross_format_function_call_upstream_stream_
                 },
                 {
                     "type": "function_call",
+                    "id": "call_auto_1",
                     "call_id": "call_auto_1",
                     "name": "get_weather",
                     "arguments": "{\"location\":\"Tokyo\"}"
@@ -1410,13 +1411,13 @@ async fn gateway_executes_openai_cli_antigravity_cross_format_upstream_stream_vi
     assert_eq!(
         response_json,
         json!({
-            "id": "resp_antigravity_cli_xfmt_123",
+            "id": "resp-local-stream",
             "object": "response",
             "status": "completed",
             "model": "claude-sonnet-4-5",
             "output": [{
                 "type": "message",
-                "id": "resp_antigravity_cli_xfmt_123_msg",
+                "id": "resp-local-stream_msg",
                 "role": "assistant",
                 "status": "completed",
                 "content": [{