Expand custom-node startup error tracking to differentiate between user-installed custom_nodes, built-in comfy_extras, and partner comfy_api_nodes. Each NODE_STARTUP_ERRORS entry now carries a 'source' field and is keyed by '<source>:<module_name>' so colliding module names across the three locations don't overwrite each other. The /custom_node_startup_errors endpoint returns errors grouped by source so the frontend/Manager can render distinct sections.
Also captures previously-missed failures from comfy_entrypoint() (phase='entrypoint').
Introduces nodes.record_node_startup_error() helper used by load_custom_node and main.execute_prestartup_script.
Adds tests-unit/node_startup_errors_test.py (6 tests) covering field shape, source mapping for each module_parent, cross-source collisions, and default fallback.
Ref: ComfyUI-Launcher#303
Amp-Thread-ID: https://ampcode.com/threads/T-019e23a1-2acc-7619-bd0e-f783d1368ef3
Co-authored-by: Amp <amp@ampcode.com>
