T24: Change ChatModelTypes.Message rawContent from Object to String

Spring AI's Content.getText() returns String. We always cast to String
on both sides. Object type gave false flexibility that would
ClassCastException at runtime.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: donald-pinckney

TASK_QUEUE.json

@@ -48,7 +48,7 @@
       "severity": "low",
       "category": "bugfix",
       "depends_on": [],
-      "status": "todo",
+      "status": "completed",
       "notes": "DABH review comment: https://github.com/temporalio/sdk-java/pull/2829/files#r3054049714. Verified Spring AI Message interface uses String, not Object."
     },
     {

temporal-spring-ai/src/main/java/io/temporal/springai/activity/ChatModelActivityImpl.java

@@ -115,9 +115,9 @@ private Prompt createPrompt(ChatModelTypes.ChatModelActivityInput input) {
 
   private org.springframework.ai.chat.messages.Message toSpringMessage(Message message) {
     return switch (message.role()) {
-      case SYSTEM -> new SystemMessage((String) message.rawContent());
+      case SYSTEM -> new SystemMessage(message.rawContent());
       case USER -> {
-        UserMessage.Builder builder = UserMessage.builder().text((String) message.rawContent());
+        UserMessage.Builder builder = UserMessage.builder().text(message.rawContent());
         if (!CollectionUtils.isEmpty(message.mediaContents())) {
           builder.media(
               message.mediaContents().stream().map(this::toMedia).collect(Collectors.toList()));
@@ -126,7 +126,7 @@ private org.springframework.ai.chat.messages.Message toSpringMessage(Message mes
       }
       case ASSISTANT ->
           AssistantMessage.builder()
-              .content((String) message.rawContent())
+              .content(message.rawContent())
               .properties(Map.of())
               .toolCalls(
                   message.toolCalls() != null
@@ -152,7 +152,7 @@ private org.springframework.ai.chat.messages.Message toSpringMessage(Message mes
               .responses(
                   List.of(
                       new ToolResponseMessage.ToolResponse(
-                          message.toolCallId(), message.name(), (String) message.rawContent())))
+                          message.toolCallId(), message.name(), message.rawContent())))
               .build();
     };
   }

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

@@ -369,7 +369,7 @@ private AssistantMessage toAssistantMessage(ChatModelTypes.Message message) {
     }
 
     return AssistantMessage.builder()
-        .content((String) message.rawContent())
+        .content(message.rawContent())
         .properties(Map.of())
         .toolCalls(toolCalls)
         .media(media)

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

@@ -82,7 +82,7 @@ public record Usage(
   /**
    * A message in the conversation.
    *
-   * @param rawContent the message content (typically a String)
+   * @param rawContent the message text content
    * @param role the role of the message author
    * @param name optional name for the participant
    * @param toolCallId tool call ID this message responds to (for TOOL role)
@@ -92,7 +92,7 @@ public record Usage(
   @JsonInclude(JsonInclude.Include.NON_NULL)
   @JsonIgnoreProperties(ignoreUnknown = true)
   public record Message(
-      @JsonProperty("content") Object rawContent,
+      @JsonProperty("content") String rawContent,
       @JsonProperty("role") Role role,
       @JsonProperty("name") String name,
       @JsonProperty("tool_call_id") String toolCallId,
@@ -101,11 +101,11 @@ public record Message(
           List<ToolCall> toolCalls,
       @JsonProperty("media") @JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
           List<MediaContent> mediaContents) {
-    public Message(Object content, Role role) {
+    public Message(String content, Role role) {
       this(content, role, null, null, null, null);
     }
 
-    public Message(Object content, List<MediaContent> mediaContents, Role role) {
+    public Message(String content, List<MediaContent> mediaContents, Role role) {
       this(content, role, null, null, null, mediaContents);
     }