Parquet: Add round-trip test for encrypted shredded variant columns

[nssalian]

Rationale

Existing tests cover shredding and encryption separately, not together. Adding this to clarify the encryption behavior on a shredded variant.

Changes

• Adds a test covering a shredded variant column written to an encrypted Parquet file: no-key read fails, keyed read round-trips, and the decrypted footer confirms the shredded layout. Test-only.
• Cleaned up some of the common artifacts into helpers from the previous tests so we don't have to duplicate.

[nssalian]

Thanks for the reviews @wombatu-kun. I cleaned up these tests and some of the older ones in the file too so we reuse with helpers.