fix: remove unused WorkflowEvent import and restore 'invalid content' retry check

- Remove unused WorkflowEvent import from migration_processor.py (fixes lint F401)
- Restore 'model produced invalid content' transient error retry in
  azure_openai_response_retry.py (regression from agent-framework 1.3.0 refactor)

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

Author: Dhanushree-Microsoft

src/processor/src/libs/agent_framework/azure_openai_response_retry.py

@@ -82,6 +82,14 @@ def _looks_like_rate_limit(error: BaseException) -> bool:
     if isinstance(status, int) and 500 <= status < 600:
         return True
 
+    # "The model produced invalid content" is a transient error from Azure OpenAI
+    # when the model output fails content/schema validation — worth retrying.
+    if any(
+        s in msg
+        for s in ["model produced invalid content", "invalid content"]
+    ):
+        return True
+
     cause = getattr(error, "__cause__", None)
     if cause and cause is not error:
         return _looks_like_rate_limit(cause)

src/processor/src/steps/migration_processor.py

@@ -32,7 +32,7 @@
 from datetime import datetime
 from typing import Any
 
-from agent_framework import Workflow, WorkflowBuilder, WorkflowEvent
+from agent_framework import Workflow, WorkflowBuilder
 
 from openai import AsyncAzureOpenAI