fix: replace Role.USER with 'user' string in test_input_observer_middleware

Role is a NewType(str) in agent-framework 1.1.1, not an enum.
Role.USER raises AttributeError at runtime.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Prachig-Microsoft

src/tests/ContentProcessorWorkflow/libs/agent_framework/test_input_observer_middleware.py

@@ -7,7 +7,7 @@
 import asyncio
 from types import SimpleNamespace
 
-from agent_framework import Message, Role
+from agent_framework import Message
 
 from libs.agent_framework.middlewares import InputObserverMiddleware
 
@@ -16,7 +16,7 @@ def test_input_observer_middleware_replaces_user_text_when_configured() -> None:
     async def _run() -> None:
         ctx = SimpleNamespace(
             messages=[
-                Message(role=Role.USER, contents=["original"]),
+                Message(role="user", contents=["original"]),
             ]
         )
 
@@ -27,7 +27,7 @@ async def _next(_context):
 
         await mw.process(ctx, _next)
 
-        assert ctx.messages[0].role == Role.USER
+        assert ctx.messages[0].role == "user"
         assert ctx.messages[0].text == "replacement"
 
     asyncio.run(_run())