fix(variant): improve malformed object field diagnostics

[cakeni]

Which issue does this PR close?

• Closes [Variant] Document and test validation of malformed shredded object fields #10619.

Rationale for this change

A named child of a shredded object is required to use the shredded Variant field Struct layout. Treating a present child with another Arrow type as a missing path would hide malformed input, so the existing InvalidArgumentError is the correct boundary.

What changes are included in this PR?

• Document the shredded-object field invariant where it is validated.
• Include the field name and actual Arrow type in the error.
• Add a malformed Int32 child regression test.

Are these changes tested?

• cargo +stable-x86_64-pc-windows-gnu test -p parquet-variant-compute --lib (348 passed)
• cargo fmt --all -- --check
• git diff --check

Are there any user-facing changes?

Malformed shredded object errors now identify the offending field and physical type. There are no public API changes.