fix: correct Bedrock SSE stream event format to match Claude API spec

- message_start: nest fields inside 'message' object with type: 'message'
- content_block_delta: add type field to data
- message_delta: add type field to data
- message_stop: remove usage field, just return type
- Extract usage from message_delta instead of message_stop

src/services/bedrockRelayService.js

@@ -343,8 +343,8 @@ class BedrockRelayService {
             res.write(`event: ${claudeEvent.type}\n`)
             res.write(`data: ${JSON.stringify(claudeEvent.data)}\n\n`)
 
-            // 提取使用统计
-            if (claudeEvent.type === 'message_stop' && claudeEvent.data.usage) {
+            // 提取使用统计 (usage is reported in message_delta per Claude API spec)
+            if (claudeEvent.type === 'message_delta' && claudeEvent.data.usage) {
               totalUsage = claudeEvent.data.usage
             }
 
@@ -576,14 +576,17 @@ class BedrockRelayService {
       return {
         type: 'message_start',
         data: {
-          type: 'message',
-          id: `msg_${Date.now()}_bedrock`,
-          role: 'assistant',
-          content: [],
-          model: this.defaultModel,
-          stop_reason: null,
-          stop_sequence: null,
-          usage: bedrockChunk.message?.usage || { input_tokens: 0, output_tokens: 0 }
+          type: 'message_start',
+          message: {
+            id: `msg_${Date.now()}_bedrock`,
+            type: 'message',
+            role: 'assistant',
+            content: [],
+            model: this.defaultModel,
+            stop_reason: null,
+            stop_sequence: null,
+            usage: bedrockChunk.message?.usage || { input_tokens: 0, output_tokens: 0 }
+          }
         }
       }
     }
@@ -603,6 +606,7 @@ class BedrockRelayService {
       return {
         type: 'content_block_delta',
         data: {
+          type: 'content_block_delta',
           index: bedrockChunk.index || 0,
           delta: bedrockChunk.delta || {}
         }
@@ -623,6 +627,7 @@ class BedrockRelayService {
       return {
         type: 'message_delta',
         data: {
+          type: 'message_delta',
           delta: bedrockChunk.delta || {},
           usage: bedrockChunk.usage || {}
         }
@@ -633,7 +638,7 @@ class BedrockRelayService {
       return {
         type: 'message_stop',
         data: {
-          usage: bedrockChunk.usage || {}
+          type: 'message_stop'
         }
       }
     }