fix(api-io): serialize MultiCombo multi_select as object config (#13484)

* fix(api-io): serialize MultiCombo multi_select as object config
* fix: remove dead code and redundant top-level keys from MultiCombo serialization
* fix: correct skip warning to mention comfy_entrypoint, remove nonexistent NODES_LIST
* fix: validate MultiCombo list values against options individually
* fix: gate multiselect validation on schema config, improve error message, add tests

---------

Co-authored-by: Ni-zav <ni-zav@users.noreply.github.com>
Co-authored-by: guill <jacob.e.segal@gmail.com>

comfy_api/latest/_io.py

@@ -395,7 +395,6 @@ class Combo(ComfyTypeIO):
 @comfytype(io_type="COMBO")
 class MultiCombo(ComfyTypeI):
     '''Multiselect Combo input (dropdown for selecting potentially more than one value).'''
-    # TODO: something is wrong with the serialization, frontend does not recognize it as multiselect
     Type = list[str]
     class Input(Combo.Input):
         def __init__(self, id: str, options: list[str], display_name: str=None, optional=False, tooltip: str=None, lazy: bool=None,
@@ -408,12 +407,14 @@ class MultiCombo(ComfyTypeI):
             self.default: list[str]
 
         def as_dict(self):
-            to_return = super().as_dict() | prune_dict({
-                "multi_select": self.multiselect,
-                "placeholder": self.placeholder,
-                "chip": self.chip,
+            # Frontend expects `multi_select` to be an object config (not a boolean).
+            # Keep top-level `multiselect` from Combo.Input for backwards compatibility.
+            return super().as_dict() | prune_dict({
+                "multi_select": prune_dict({
+                    "placeholder": self.placeholder,
+                    "chip": self.chip,
+                }),
             })
-            return to_return
 
 @comfytype(io_type="IMAGE")
 class Image(ComfyTypeIO):