Address review feedback from Kosinkadink:
1. Move JSON loading logic from nodes.py into NodeReplaceManager as
load_from_json() method for better encapsulation and testability
2. Tests now exercise the real NodeReplaceManager (no duplicated logic)
3. Defer `import nodes` in apply_replacements to avoid torch at import
4. nodes.py call site simplified to one line:
PromptServer.instance.node_replace_manager.load_from_json(...)
Skip entries in the prompt dict that don't contain a class_type key
in apply_replacements(), preventing crashes on metadata or non-node
entries.
FixesComfy-Org/ComfyUI#12517