temporal-spring-ai: preserve Usage and RateLimit in ChatResponse metadata

ActivityChatModel.toResponse now rehydrates Usage and RateLimit onto the
ChatResponseMetadata it returns to workflow code, not just the model
name. The activity side (ChatModelActivityImpl) already serialized
these into the output record; they were being silently discarded when
the workflow side rebuilt the ChatResponse.

Usage is rehydrated as a Spring AI DefaultUsage(promptTokens,
completionTokens, totalTokens). RateLimit is an interface with no
public default impl in spring-ai-model, so we return an anonymous
implementation backed by the fields from the activity output record.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: donald-pinckney

temporal-spring-ai/src/main/java/io/temporal/springai/model/ActivityChatModel.java

@@ -12,6 +12,9 @@
 import java.util.stream.Collectors;
 import org.springframework.ai.chat.messages.*;
 import org.springframework.ai.chat.metadata.ChatResponseMetadata;
+import org.springframework.ai.chat.metadata.DefaultUsage;
+import org.springframework.ai.chat.metadata.RateLimit;
+import org.springframework.ai.chat.metadata.Usage;
 import org.springframework.ai.chat.model.ChatModel;
 import org.springframework.ai.chat.model.ChatResponse;
 import org.springframework.ai.chat.model.Generation;
@@ -346,11 +349,70 @@ private ChatResponse toResponse(ChatModelTypes.ChatModelActivityOutput output) {
 
     var builder = ChatResponse.builder().generations(generations);
     if (output.metadata() != null) {
-      builder.metadata(ChatResponseMetadata.builder().model(output.metadata().model()).build());
+      builder.metadata(toResponseMetadata(output.metadata()));
     }
     return builder.build();
   }
 
+  private ChatResponseMetadata toResponseMetadata(
+      ChatModelTypes.ChatModelActivityOutput.ChatResponseMetadata md) {
+    ChatResponseMetadata.Builder b = ChatResponseMetadata.builder().model(md.model());
+    Usage usage = toUsage(md.usage());
+    if (usage != null) {
+      b.usage(usage);
+    }
+    RateLimit rateLimit = toRateLimit(md.rateLimit());
+    if (rateLimit != null) {
+      b.rateLimit(rateLimit);
+    }
+    return b.build();
+  }
+
+  private Usage toUsage(ChatModelTypes.ChatModelActivityOutput.ChatResponseMetadata.Usage u) {
+    if (u == null) {
+      return null;
+    }
+    return new DefaultUsage(u.promptTokens(), u.completionTokens(), u.totalTokens());
+  }
+
+  private RateLimit toRateLimit(
+      ChatModelTypes.ChatModelActivityOutput.ChatResponseMetadata.RateLimit r) {
+    if (r == null) {
+      return null;
+    }
+    return new RateLimit() {
+      @Override
+      public Long getRequestsLimit() {
+        return r.requestLimit();
+      }
+
+      @Override
+      public Long getRequestsRemaining() {
+        return r.requestRemaining();
+      }
+
+      @Override
+      public java.time.Duration getRequestsReset() {
+        return r.requestReset();
+      }
+
+      @Override
+      public Long getTokensLimit() {
+        return r.tokenLimit();
+      }
+
+      @Override
+      public Long getTokensRemaining() {
+        return r.tokenRemaining();
+      }
+
+      @Override
+      public java.time.Duration getTokensReset() {
+        return r.tokenReset();
+      }
+    };
+  }
+
   private AssistantMessage toAssistantMessage(ChatModelTypes.Message message) {
     List<AssistantMessage.ToolCall> toolCalls = List.of();
     if (!CollectionUtils.isEmpty(message.toolCalls())) {