docs(cli): mirror non-text content guard in sync README snippets

Update the illustrative handler snippets in the sync READMEs to use the
same isinstance(content, TextContent) guard as the generated code, so the
docs don't teach the crashing params.content.content pattern.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: declan-scale

src/agentex/lib/cli/templates/sync-openai-agents-local-sandbox/README.md.j2

@@ -262,7 +262,10 @@ Add sophisticated response generation:
 @acp.on_message_send
 async def handle_message_send(params: SendMessageParams):
     # Analyze input
-    user_message = params.content.content
+    content = params.content
+    if not isinstance(content, TextContent):
+        return TextContent(author="agent", content="Sorry, I can only handle text messages right now.")
+    user_message = content.content
 
     # Generate response
     response = await generate_intelligent_response(user_message)

src/agentex/lib/cli/templates/sync-openai-agents/README.md.j2

@@ -251,7 +251,10 @@ Add sophisticated response generation:
 @acp.on_message_send
 async def handle_message_send(params: SendMessageParams):
     # Analyze input
-    user_message = params.content.content
+    content = params.content
+    if not isinstance(content, TextContent):
+        return TextContent(author="agent", content="Sorry, I can only handle text messages right now.")
+    user_message = content.content
 
     # Generate response
     response = await generate_intelligent_response(user_message)

src/agentex/lib/cli/templates/sync-pydantic-ai/README.md.j2

@@ -251,7 +251,10 @@ Add sophisticated response generation:
 @acp.on_message_send
 async def handle_message_send(params: SendMessageParams):
     # Analyze input
-    user_message = params.content.content
+    content = params.content
+    if not isinstance(content, TextContent):
+        return TextContent(author="agent", content="Sorry, I can only handle text messages right now.")
+    user_message = content.content
 
     # Generate response
     response = await generate_intelligent_response(user_message)

src/agentex/lib/cli/templates/sync/README.md.j2

@@ -251,7 +251,10 @@ Add sophisticated response generation:
 @acp.on_message_send
 async def handle_message_send(params: SendMessageParams):
     # Analyze input
-    user_message = params.content.content
+    content = params.content
+    if not isinstance(content, TextContent):
+        return TextContent(author="agent", content="Sorry, I can only handle text messages right now.")
+    user_message = content.content
 
     # Generate response
     response = await generate_intelligent_response(user_message)