fix(hardening): address PR #157 review findings

Critical fixes:
- node_runtime.py: change outer handler in _load_module() from except Exception to except BaseException with sys.modules.pop() cleanup, preventing non-Exception BaseExceptions (SystemExit, KeyboardInterrupt) from violating the no-raise contract
- app.py: remove incorrect reassignment of base_node_class_mappings after custom node import; keep it as pre-custom-node baseline so WorkflowPlanner correctly treats extras/custom nodes as NODE_CLASS_MAPPINGS dict lookups instead of direct imports
- Regenerate stale test fixtures (text-to-image.py, upscale-model-loader.py) from current source: updated _load_module() with exec failure cleanup pattern, 3-marker structural verification, and try/finally sys.path restoration in import_custom_nodes()

Suggestions addressed:
- node_runtime.py: _is_comfyui_directory() now checks 3 structural markers (nodes.py, main.py, comfy/) instead of just nodes.py
- generator/render.py: added None guard for getattr on generated_helpers.__all__ items with log warning for missing helpers
- test_import_path_resolution.py: fixed test_sys_path_restored_on_crash to target the server module specifically after sys.path manipulation, so the try/finally restoration path is actually exercised

Nits addressed:
- .gitignore: added missing trailing newline

New test coverage (tests/test_app_base_mappings.py):
- base_node_class_mappings unchanged after custom node import
- deep copy independence of base_node_class_mappings
- render output contains hardened _load_module with except BaseException pattern

comfyui_to_python/node_runtime.py

@@ -9,10 +9,18 @@ log = logging.getLogger(__name__)
 
 
 def _is_comfyui_directory(path: str) -> bool:
-    """Verify a directory has ComfyUI structural markers (nodes.py)."""
+    """Verify a directory has ComfyUI structural markers.
+
+    Checks for nodes.py, main.py, and the comfy/ subdirectory to raise
+    the bar against spoofing via a directory with only a single marker file.
+    """
     if not os.path.isdir(path):
         return False
-    return os.path.isfile(os.path.join(path, "nodes.py"))
+    return (
+        os.path.isfile(os.path.join(path, "nodes.py"))
+        and os.path.isfile(os.path.join(path, "main.py"))
+        and os.path.isdir(os.path.join(path, "comfy"))
+    )
 
 
 def _load_module(module_name: str, filepath: str) -> Any:
@@ -43,8 +51,9 @@ def _load_module(module_name: str, filepath: str) -> Any:
             sys.modules.pop(module_name, None)
             raise
         return mod
-    except Exception as e:
+    except BaseException as e:
         log.debug("Failed to load %s from %s: %s", module_name, filepath, e)
+        sys.modules.pop(module_name, None)  # Also clean up on edge-case exceptions
         return None