Named callback arguments could default to `no_update` if not provided

Apologies if this is a duplicate, I tried a search and didn't see anything.

Is your feature request related to a problem? Please describe.

Imagine a callback with many outputs, using named keyword arguments.

@app.callback(
    output=dict(out_1=..., out_2=..., out_3=..., error_div=...),
    input=...
)
def my_callback(...):
   # I have to return a dict with _all_ outputs defined even if several are no_update
   # e.g., I have an error and I only want to report the error to the user
   return dict(out_1=no_update, out_2=no_update, out_3=no_update, error_div=...)

Describe the solution you'd like

It would be nice if, instead, if a key is not present it is assumed to be no_update by default, e.g. in the above example we could just return:

    return dict(error_div=...)

Currently, this would raise a schema error.

Describe alternatives you've considered
N/A

Additional context
N/A

Start at the callback output handling and the schema validation that currently rejects dictionaries missing declared output keys. Define the behavior so omitted named outputs are treated as no_update, then verify that returning only error_div works while invalid or unexpected keys still produce the appropriate schema error.