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/handlers/admin/provider/pool/runtime/writes.rs

@@ -886,7 +886,7 @@ mod tests {
         record_admin_provider_pool_stream_timeout(&runner, "provider-1", "key-4", &pool_config)
             .await;
 
-        let runtime = read_admin_provider_pool_runtime_state(
+        let mut runtime = read_admin_provider_pool_runtime_state(
             &runner,
             "provider-1",
             &key_ids,
@@ -894,6 +894,25 @@ mod tests {
             None,
         )
         .await;
+        for _ in 0..20 {
+            if runtime
+                .cooldown_reason_by_key
+                .get("key-4")
+                .map(String::as_str)
+                == Some("stream_timeout_x2")
+            {
+                break;
+            }
+            tokio::time::sleep(std::time::Duration::from_millis(10)).await;
+            runtime = read_admin_provider_pool_runtime_state(
+                &runner,
+                "provider-1",
+                &key_ids,
+                &pool_config,
+                None,
+            )
+            .await;
+        }
 
         assert_eq!(
             runtime