feat: add type-signature-based fallback for unregistered nodes

GenericNodeExtractor (previously a no-op) now inspects
RETURN_TYPES to detect MODEL loaders and CONDITIONING
encoders in nodes not registered in NODE_EXTRACTORS.

- Propagate return_types from the hook layer through the
  registry to GenericNodeExtractor.extract() and update().
- MODEL detection: scan ckpt_name/unet_name/model_path/
  model_name/gguf_name fields, validate by extension.
- CONDITIONING detection: scan text/clip_l/t5xxl/prompt
  fields, store prompt text and conditioning tensor.
- _fill_missing_metadata also checks node_cache, so
  GenericNodeExtractor-handled nodes survive cache.

tests/metadata_collector/test_metadata_collector.py

@@ -30,10 +30,10 @@ def test_metadata_hook_installs_and_traces_execution(monkeypatch, metadata_regis
 
     calls = []
 
-    def record_stub(self, node_id, class_type, inputs, outputs):
+    def record_stub(self, node_id, class_type, inputs, outputs, return_types=None):
         calls.append(("record", node_id, class_type, inputs))
 
-    def update_stub(self, node_id, class_type, outputs):
+    def update_stub(self, node_id, class_type, outputs, return_types=None):
         calls.append(("update", node_id, class_type, outputs))
 
     monkeypatch.setattr(MetadataRegistry, "record_node_execution", record_stub)