fix(nodes): better defaults parsing and error handling

[psychedelicious]

Summary

Fixes and better error handling for invalid invocation field default values.

Related Issues / Discussions

Discord thread: https://discord.com/channels/1020123559063990373/1149506274971631688/1374029733423087636

QA Instructions

Using this repo as a test https://github.com/mickr777/imagetoasciiimage

Before

• No error in launcher/python terminal, just looks fine
• UI hangs at loading screen
• Error like this in the JS console of browser
  

After

• Big error in launcher/python terminal
• UI does not hang
• If, somehow, the invocation gets past the python safeguards, instead of hanging the UI, the error is logged and the UI loads.

Example terminal error:

[2025-05-20 08:30:45,802]::[InvokeAI]::ERROR --> Failed to load node pack imagetoasciiimage (may have partially loaded):
Traceback (most recent call last):
  File "/home/bat/Documents/Code/InvokeAI/invokeai/app/invocations/baseinvocation.py", line 530, in validate_field_default
    TempDefaultValidator.model_validate({field_name: orig_default})
  File "/home/bat/Documents/Code/InvokeAI/.venv/lib/python3.12/site-packages/pydantic/main.py", line 703, in model_validate
    return cls.__pydantic_validator__.validate_python(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for ImageToAAInvocation
local_font
  Input should be 'None' [type=literal_error, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.11/v/literal_error

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/bat/Documents/Code/InvokeAI/invokeai/app/invocations/load_custom_nodes.py", line 69, in load_custom_nodes
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 995, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/home/bat/invokeai-4.0.0/nodes/imagetoasciiimage/__init__.py", line 2, in <module>
    from .i2aa_anyfont import ImageToAAInvocation
  File "/home/bat/invokeai-4.0.0/nodes/imagetoasciiimage/i2aa_anyfont.py", line 116, in <module>
    @invocation(
     ^^^^^^^^^^^
  File "/home/bat/Documents/Code/InvokeAI/invokeai/app/invocations/baseinvocation.py", line 594, in wrapper
    validate_field_default(cls.__name__, field_name, invocation_type, annotation, field_info)
  File "/home/bat/Documents/Code/InvokeAI/invokeai/app/invocations/baseinvocation.py", line 532, in validate_field_default
    raise InvalidFieldError(
invokeai.app.invocations.baseinvocation.InvalidFieldError: Default value for field "local_font" on invocation "I2AA_AnyFont" is invalid, 1 validation error for ImageToAAInvocation
local_font
  Input should be 'None' [type=literal_error, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.11/v/literal_error

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)