Fix aggregate deserial function signature

[eeeebbbbrrrr]

Summary

• generate aggregate deserial functions with PostgreSQL's required (bytea, internal) -> internal signature
• update Aggregate::deserial to mirror that contract without a spurious current-state argument or PgBox conversion
• add macro-level and live PostgreSQL regression coverage for a parallel aggregate with internal state

Fixes #2351

Testing

• cargo test -p pgrx-sql-entity-graph
• cargo pgrx test pg18 aggregate_parallel_internal_deserial
• cargo check -p pgrx --no-default-features --features pg18
• cargo check -p pgrx-unit-tests --no-default-features --features pg18,pg_test
• cargo check --manifest-path pgrx-examples/aggregate/Cargo.toml --no-default-features --features pg18
• cargo clippy -p pgrx-sql-entity-graph -- -D warnings

[eeeebbbbrrrr]

I am not sure that the attempt here is actually correct or in the spirit of pgrx. Having a function signature that takes Internal as an argument I believe is incorrect for this API. I need to study this more.