Fix auth routing in samples 06/11: api_key -> credential for Azure OpenAI

Both samples passed a bearer token provider via api_key= which caused the
client to route to api.openai.com instead of Azure OpenAI, resulting in
401 Unauthorized. Changed to credential= which correctly triggers Azure
routing and picks up AZURE_OPENAI_ENDPOINT from the environment.

- samples/azure_functions/11_workflow_parallel/function_app.py: 1 fix
- samples/durabletask/06_multi_agent_orchestration_conditionals/worker.py: 2 fixes
- Re-enable 4 parallel workflow tests and 1 conditional branching test

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

python/samples/04-hosting/azure_functions/11_workflow_parallel/function_app.py

@@ -363,7 +363,7 @@ def _create_workflow() -> Workflow:
 
     chat_client = OpenAIChatCompletionClient(
         model=os.environ["AZURE_OPENAI_MODEL"],
-        api_key=get_bearer_token_provider(credential, "https://cognitiveservices.azure.com/.default"),
+        credential=get_bearer_token_provider(credential, "https://cognitiveservices.azure.com/.default"),
     )
 
     # Create agents for parallel analysis

python/samples/04-hosting/durabletask/06_multi_agent_orchestration_conditionals/worker.py

@@ -70,7 +70,7 @@ def create_spam_agent() -> "Agent":
     return Agent(
         client=OpenAIChatCompletionClient(
             model=os.environ["AZURE_OPENAI_MODEL"],
-            api_key=get_async_bearer_token_provider(
+            credential=get_async_bearer_token_provider(
                 AsyncAzureCliCredential(), "https://cognitiveservices.azure.com/.default"
             ),
         ),
@@ -88,7 +88,7 @@ def create_email_agent() -> "Agent":
     return Agent(
         client=OpenAIChatCompletionClient(
             model=os.environ["AZURE_OPENAI_MODEL"],
-            api_key=get_async_bearer_token_provider(
+            credential=get_async_bearer_token_provider(
                 AsyncAzureCliCredential(), "https://cognitiveservices.azure.com/.default"
             ),
         ),