feat: add session execution mode with WorkflowSession and WorkflowSessionRuntime

Add session-mode code generation that produces a reusable WorkflowSession class
instead of a simple oneshot script. The session mode generates code with an
in-class run/close lifecycle for warm ComfyUI reuse.

Also refactors render.py to extract shared code sections into common methods,
reducing duplication between oneshot and session renderers.

Adds WorkflowSessionRuntime with cleanup policies (per_run, session, manual)
and reset_every_n_runs support, plus 72 new tests covering session rendering,
runtime session, and session export pipeline.

comfyui_to_python/generator/model.py

@@ -1,4 +1,5 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
+from typing import Literal
 
 
 @dataclass(frozen=True)
@@ -10,3 +11,5 @@ class GenerationPlan:
     metadata_workflow_data: dict | None
     queue_size: int
     custom_nodes: bool
+    execution_mode: Literal["oneshot", "session"] = field(default="oneshot")
+    executed_variables: dict[str, str] = field(default_factory=dict)