fix(predict): materialize default_factory for input fields - #10792

📝 Changes Description

Predict populated missing input defaults only when FieldInfo.default was not PydanticUndefined. For an input declared with default_factory, .default is undefined, so the factory was never called: the field was absent from the prompt, and the module logged Not all input fields were provided ... Missing: ['context'] even though the field had a default. The output-field path already materializes factories through get_default(call_default_factory=True).

class Sig(dspy.Signature):
    context: list[str] = dspy.InputField(default_factory=lambda: ["DEFAULT_CONTEXT"])
    question: str = dspy.InputField()
    answer: str = dspy.OutputField()

dspy.configure(lm=spy_lm)
Predict(Sig)(question="test")  # prompt omits context; warning fires

This change populates missing inputs with FieldInfo.is_required() and materializes factory fields via get_default(call_default_factory=True), passing the raw input values supplied so far as validated_data for factories that accept an input dict (DSPy does not validate inputs, so this mirrors the available values rather than pydantic model validation). Literal defaults keep their existing value and identity; required-field warnings, optional-annotation behavior, and output defaults are unchanged. The unused PydanticUndefined import is removed.

A related PR (#10034) made an equivalent Predict change in a stacked feature branch (isaac/rlm-namespace-collisions); it is not in current main, which still has the old code (verified against c8176a8).

✅ Contributor Checklist

• Pre-Commit checks are passing (locally: pytest tests/predict 614 passed, ruff check clean, ty check with the same 16 pre-existing diagnostics as base)
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⚠️ Warnings

Tests use the existing spy_lm helper and the default adapter; the fix is covered through Predict._forward_preprocess, which is shared by forward and async paths.