Harden variant decoding

[steveloughran]

Describe the enhancement requested

org.apache.parquet.variant.Variant is brittle against malformed variants, whereas the arrow variant is very strict and includes an optional deep validate phase

• Harden the java implementation, where it doesn't impose any performance penalty
• add tests which can also generate malformed files to add to the parquet test files

The work in #3481 should include some of this or this should be an immediate followon (preferred)

Component(s)

parquet-variant