GPT-5, GPT-5-mini support

Improvements to model name resolution
Improved instructions for multi-step workflows when continuation is available
Improved instructions for chat tool
Improved preferred model resolution, moved code from registry -> each provider
Updated tests

tools/workflow/workflow_mixin.py

@@ -663,13 +663,13 @@ class BaseWorkflowMixin(ABC):
                 self._current_model_name = None
                 self._model_context = None
 
+            # Handle continuation
+            continuation_id = request.continuation_id
+
             # Adjust total steps if needed
             if request.step_number > request.total_steps:
                 request.total_steps = request.step_number
 
-            # Handle continuation
-            continuation_id = request.continuation_id
-
             # Create thread for first step
             if not continuation_id and request.step_number == 1:
                 clean_args = {k: v for k, v in arguments.items() if k not in ["_model_context", "_resolved_model_name"]}