Preserve invalid scalar embed params

[fishtreesugar]

What changed

• Preserve scalar params during schema transformation so Ecto can handle invalid embed input.
• Add regression coverage for a scalar item inside embeds_many.
• Add regression coverage for a scalar value passed to embeds_one.

Why

A scalar item supplied to an embedded schema has no fields to map. It previously fell through to transform_from_struct/3, where get_params_field_value/3 crashed before Ecto could validate the input.

Returning the scalar unchanged allows cast_embed to produce the expected type validation error instead of raising.

Impact

Invalid scalar values for embeds_one and scalar elements inside embeds_many now return normal changeset errors.

Validation

• mix format --check-formatted lib/params.ex test/strukt_test.exs
• mix test test/strukt_test.exs — 51 tests passed
• git diff --check