🐛 Derive socket requiredness from a model field's default

[elinscott]

Summary

A task taking a dataclass or a pydantic model as input refused to run unless the caller filled in every field, defaults included. Given

@dataclass
class Base:
    nelec: int
    tot_magnetization: Optional[int] = None
    ecut: float = 40.0

@task
def consume(settings: Base) -> int:
    return settings.nelec

a caller who set only nelec was told settings.tot_magnetization was missing, and ecut escaped the same complaint only because its default is not None. SocketSpecAPI.from_model copied each field's default onto the child spec but never derived requiredness from it, so meta.required kept its True default. The function-signature path and the TypedDict path already derive requiredness; the model paths now do too.

Changes

• Mark a model field's socket optional when the field declares a default, in all four branches of from_model
• Treat a default_factory as a default
• Apply requiredness to namespace fields as well, though defaults are still set only on leaves. A field like sub: Optional[Sub] = None may be omitted like any other; a namespace simply has no place to keep the default value, and its requiredness is the only thing that says the caller may skip it.
• The flag is set to False rather than left as None. If we left as None, merge_meta lets a True base survive it, and SocketMeta.to_dict omits it so from_dict restores the True default. Only False states optionality in a way that survives a round trip

Not covered

Three other paths still set a leaf default without touching requiredness: namespace(b=(int, 5)), set_default, and the mapping default of a namespace parameter. They are outside from_model and therefore outside this fix; a downstream consumer that exempts defaulted sockets from its own missing-input check still needs that exemption for them.

Closes #172

[codecov]

Codecov Report

❌ Patch coverage is 97.70115% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.77%. Comparing base (8d85e61) to head (2437e1e).

Files with missing lines	Patch %	Lines
tests/test_socket_spec.py	97.18%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #173      +/-   ##
==========================================
+ Coverage   89.68%   89.77%   +0.09%     
==========================================
  Files          81       81              
  Lines        8984     9069      +85     
==========================================
+ Hits         8057     8142      +85     
  Misses        927      927              

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