Fix chat middleware: add streaming support, terminate flag, and check only last message (#2120)

This commit fixes three issues in the security_filter_middleware:

1. Missing context.terminate flag - Without this, middleware continues processing after setting blocked response
2. No streaming support - When context.is_streaming is True, middleware now returns async generator with ChatResponseUpdate
3. Checks all messages - Changed to check only context.messages[-1] (most recent user message) instead of iterating through conversation history

Changes:
- Added AsyncIterable import
- Added ChatResponseUpdate and TextContent imports
- Modified security_filter_middleware to handle both streaming and non-streaming modes
- Added context.terminate = True to properly stop execution
- Changed message checking logic to only inspect the last user message

Co-authored-by: Victor Dibia <chuvidi2003@gmail.com>
Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/samples/getting_started/devui/weather_agent_azure/agent.py

@@ -3,7 +3,7 @@
 
 import logging
 import os
-from collections.abc import Awaitable, Callable
+from collections.abc import AsyncIterable, Awaitable, Callable
 from typing import Annotated
 
 from agent_framework import (
@@ -11,8 +11,10 @@ from agent_framework import (
     ChatContext,
     ChatMessage,
     ChatResponse,
+    ChatResponseUpdate,
     FunctionInvocationContext,
     Role,
+    TextContent,
     chat_middleware,
     function_middleware,
     ai_function
@@ -37,28 +39,42 @@ async def security_filter_middleware(
     next: Callable[[ChatContext], Awaitable[None]],
 ) -> None:
     """Chat middleware that blocks requests containing sensitive information."""
-    # Block requests with sensitive information
     blocked_terms = ["password", "secret", "api_key", "token"]
 
-    for message in context.messages:
-        if message.text:
-            message_lower = message.text.lower()
-            for term in blocked_terms:
-                if term in message_lower:
-                    # Override the response without calling the LLM
+    # Check only the last message (most recent user input)
+    last_message = context.messages[-1] if context.messages else None
+    if last_message and last_message.role == Role.USER and last_message.text:
+        message_lower = last_message.text.lower()
+        for term in blocked_terms:
+            if term in message_lower:
+                error_message = (
+                    "I cannot process requests containing sensitive information. "
+                    "Please rephrase your question without including passwords, secrets, "
+                    "or other sensitive data."
+                )
+
+                if context.is_streaming:
+                    # Streaming mode: return async generator
+                    async def blocked_stream() -> AsyncIterable[ChatResponseUpdate]:
+                        yield ChatResponseUpdate(
+                            contents=[TextContent(text=error_message)],
+                            role=Role.ASSISTANT,
+                        )
+
+                    context.result = blocked_stream()
+                else:
+                    # Non-streaming mode: return complete response
                     context.result = ChatResponse(
                         messages=[
                             ChatMessage(
                                 role=Role.ASSISTANT,
-                                text=(
-                                    "I cannot process requests containing sensitive information. "
-                                    "Please rephrase your question without including passwords, secrets, "
-                                    "or other sensitive data."
-                                ),
+                                text=error_message,
                             )
                         ]
                     )
-                    return
+
+                context.terminate = True
+                return
 
     await next(context)