fix(ai-amazon-bedrock): emit finish part after metadata event to preserve token usage in streaming

.changeset/fix-bedrock-streaming-cached-tokens.md

@@ -0,0 +1,5 @@
+---
+"@effect/ai-amazon-bedrock": patch
+---
+
+Fix streaming responses losing all token usage data (including cached input tokens). The finish part was being emitted during the `messageStop` event before the `metadata` event had populated token counts, causing `inputTokens`, `outputTokens`, `totalTokens`, and `cachedInputTokens` to be uninitialized. The finish part is now deferred until both `messageStop` and `metadata` have been received, matching the Bedrock ConverseStream API event ordering.

packages/ai/amazon-bedrock/src/AmazonBedrockLanguageModel.ts

@@ -638,12 +638,27 @@ const makeStreamResponse: (
 
     let trace: ConverseTrace | undefined = undefined
     let cacheWriteInputTokens: number | undefined = undefined
+    let finishReason: Response.FinishReason | undefined = undefined
+    let hasMetadata = false
     const usage: Mutable<typeof Response.Usage.Encoded> = {
       inputTokens: undefined,
       outputTokens: undefined,
       totalTokens: undefined
     }
 
+    const tryEmitFinish = (parts: Array<Response.StreamPartEncoded>) => {
+      if (finishReason !== undefined && hasMetadata) {
+        parts.push({
+          type: "finish",
+          reason: finishReason,
+          usage,
+          metadata: {
+            bedrock: { trace, usage: { cacheWriteInputTokens } }
+          }
+        })
+      }
+    }
+
     return stream.pipe(
       Stream.mapEffect(Effect.fnUntraced(function*(event) {
         const parts: Array<Response.StreamPartEncoded> = []
@@ -659,16 +674,8 @@ const makeStreamResponse: (
           }
 
           case "messageStop": {
-            const reason = InternalUtilities.resolveFinishReason(event.messageStop.stopReason)
-            parts.push({
-              type: "finish",
-              reason,
-              usage,
-              metadata: {
-                bedrock: { trace, usage: { cacheWriteInputTokens } }
-              }
-            })
-
+            finishReason = InternalUtilities.resolveFinishReason(event.messageStop.stopReason)
+            tryEmitFinish(parts)
             break
           }
 
@@ -890,6 +897,8 @@ const makeStreamResponse: (
             if (Predicate.isNotUndefined(event.metadata.trace)) {
               trace = event.metadata.trace
             }
+            hasMetadata = true
+            tryEmitFinish(parts)
             break
           }