fix(test): update LlamaStackAsLibraryClient initialization tests after removing initialize method

The recent refactor (3778a4c3) introduced automatic initialization for
  LlamaStackAsLibraryClient but the unit tests were expecting manual initalization and _is_initialized.
  This caused test failure.

  Changes:
  - Update test assertions to check route_impls is not None instead of _is_initialized
  - Add proper mocking in tests to avoid external provider dependencies
  - Maintain test coverage for automatic initialization behavior
  - Ensure backward compatibility testing for deprecated initialize() method

Signed-off-by: Mustafa Elbehery <melbeher@redhat.com>

tests/integration/fixtures/common.py

@@ -256,7 +256,6 @@ def instantiate_llama_stack_client(session):
         provider_data=get_provider_data(),
         skip_logger_removal=True,
     )
-    # Client is automatically initialized during construction
     return client
 
 

tests/integration/non_ci/responses/fixtures/fixtures.py

@@ -113,7 +113,6 @@ def openai_client(base_url, api_key, provider):
             raise ValueError(f"Invalid config for Llama Stack: {provider}, it must be of the form 'stack:<config>'")
         config = parts[1]
         client = LlamaStackAsLibraryClient(config, skip_logger_removal=True)
-        # Client is automatically initialized during construction
         return client
 
     return OpenAI(