[autoCAT6]

Rationale for this change

Writing an AES_GCM_CTR_V1 file with a plaintext footer currently records AES_GCM_V1 in the footer metadata. Readers then select GCM for the CTR data pages and fail to read them. This also reproduces with arrow-go v18.8.0.

Related to #566 and apache/arrow#54884.

What changes are included in this PR?

Serialize the configured file algorithm directly, preserving its AAD settings. Footer signing is unchanged.

Add a generated-file regression for both algorithms, encrypted and plaintext footers, and stored or externally supplied AAD prefixes. It checks serialized metadata and round-trip values, plus rejection of a corrupted plaintext-footer signature.

Are these changes tested?

The regression fails on the original code in all three CTR/plaintext cases and passes with the fix.

• All 18 Parquet packages pass with -race -tags assert. The focused regression also passes with assert,noasm.
• Files from this commit read correctly with arrow-go v18.8.0, the C++ reader from GH-51669: [C++][Parquet] Preserve cipher in plaintext footer metadata arrow#54884, and parquet-java 1.18.1, including all column values.
• Vet and lint pass for the changed packages. Apache RAT passes. Broader vet/lint checks report only the existing BitRun benchmark warning and three schema QF1001 suggestions, also reproduced on the unchanged base.

Are there any user-facing changes?

New CTR files with plaintext footers declare the correct cipher and can be read back. No API changes.

AI was used for the implementation, tests and PR description.