adapter: Add a binary encoding for int2vector

[mgree]

Motivation

When writing #35324, I ran into an issue where testdrive crashed because my query was returning an int2vector.

Description

Postgres serializes an int2vector like a 1-dimensional array with a 0 lower bound.

NB that int2vector should be non-nullable, but I copied the array nullability logic rather than asserting as much.

Verification

New test in int2vector_binary.td.

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[ggevay]

LGTM

(Tons of Nightly failures, but I guess unrelated.)

[bosconi]

bugbot run

[cursor]

PR Summary

Medium Risk
Adds a new pgwire binary encoding path for int2vector, which could affect client interoperability and system catalog queries that return pg_index.indkey. Scope is small but touches wire-format serialization and testdrive decoding.

Overview
Enables binary pgwire encoding for Value::Int2Vector by serializing it in the same shape as a 1-D array with a 0 lower bound (and marks SqlScalarType::Int2Vector as binary-encodable).

Updates testdrive result decoding to treat INT2_VECTOR like INT2_ARRAY, and adds a new testdrive case (int2vector_binary.td) that validates pg_index.indkey can be fetched in binary and cast to text.

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