feat(builder): hidden_field alias for Rails FormBuilder migration

[mhenrixon]

Summary

form.hidden_field(:accepted_terms_document_id, value: TermsDocument.current&.id) raised:

NoMethodError:
  Try including `Phlex::Rails::Helpers::HiddenField` in Forms::Form, like so:
  ...

Forms::Form exposes the PascalCase escape hatch f.Hidden(:name) but no snake_case Rails FormBuilder API, so straight hidden_field migrations from ActionView hit a NoMethodError.

This adds hidden_field to PhlexForms::Builder as a snake_case alias that delegates to the same model-bound Field#hidden path as Hidden. An explicit value: still wins over the model's current value (kwargs merge order in Field#hidden), so f.hidden_field(:token, value: x) ports over verbatim. Because it lives on the shared Builder mixin, both Forms::Form and FieldsForBuilder (nested fields_for) get it.

Test Coverage

• hidden_field renders a scoped hidden input honoring an explicit value: override (name="user[accepted_terms_document_id]", value="42")
• binds the value from the model when none is passed (value="abc123")
• renders under the plain theme too (bare <input type="hidden">) — theme parity

Verification

• bundle exec rubocop lib spec — clean
• bundle exec rspec — 165 passed, 1 pending

Deviations & judgment calls

• Reused the existing path, no new value logic. Field#hidden already reads value: field_value from the model and lets a caller value: in the splat win. hidden_field delegates to it unchanged — no duplicate value-handling.
• Left the LegacyFormMethod cop unchanged. It still nudges hidden_field → Hidden. The method now runs (enables migration) while the cop keeps steering hosts toward the idiomatic API over time — a lint nudge and a working method are complementary, not contradictory.
• Scoped to hidden_field only — the one reported failure — not the whole Rails *_field family. The cop's INPUT_TYPE_METHODS / OTHER_LEGACY_METHODS maps enumerate the rest; if hosts hit more NoMethodErrors mid-migration, those are the follow-up list.
• Added to the Builder mixin, not just Forms::Form, so nested fields_for builders get it for free — both include the mixin.