Add property tests for v3 types

[tobyhede]

Summary

Add unit and property-based tests for eql_v3. The current per-variant Rust/SQLx suites are solid integration tests, but they exercise fixed payloads. Property tests would let us assert correctness across a generated input space — e.g. that eql_v2.eq agrees with plaintext equality for any pair of int4 values.

Background

Raised by @coderdan during review of #225 (feat: eql_v2_int4 variant family):

The tests in this file are great as integration tests. I reckon we could consider adding unit and property tests, too.

EQL does not currently have a property-testing harness set up, so this is tracked as follow-up work rather than blocking the PR.

Proposed approach

Use a property-testing crate (quickcheck or proptest) with Arbitrary-derived encrypted-payload structs, comparing SQL operator results against the plaintext oracle. Sketch from the review:

mod eql_v2 {
    #[derive(Arbitrary)]
    struct Int4Eq {
        // -- glossing over details
    }
}

// -- a bit approximate
fn sql_compare(a: &Int4Eq, b: &Int4Eq) -> bool {
    let response = get_conn()
        .query("SELECT eql_v2.eq($1, $2)", &[&a, &b])
        .expect("Failed to execute query");

    let row = response.first().expect("Expected a result");

    row.get(0)
}

quickcheck! {
    fn int4_eq(x: eql_v2::Int4Eq, y: eql_v2::Int4Eq) -> bool {
        sql_compare(&x, &y) == (x.plaintext == y.plaintext)
    }
}

Scope

• Set up a property-testing harness in tests/sqlx/ (crate choice, Arbitrary impls for encrypted int4 payloads, a DB-connection helper usable from property tests).
• Equality oracle: eql_v2.eq / eql_v2.neq agree with plaintext == / != for eql_v2_int4_eq.
• Ordering oracle: <, <=, >, >= (and eql_v2.ord_term sort order) agree with plaintext numeric order for eql_v2_int4_ord / eql_v2_int4_ord_ore.
• Unit tests for edge cases (NULL handling, blocker functions raising, CHECK-constraint rejection of malformed payloads).
• Consider generalising the harness so other encrypted types can reuse it later.