JSON columns: opt-in byte-exact round-trip (preserve object key order)

[JJJ]

Surfaced by the both-engine CI matrix (#230; predicted by #220).

JsonColumnTest::test_array_roundtrips_through_json_column fails on the mysql:8.4 (and mysql:5.7) CI leg while passing on MariaDB. A value stored through a JSON column does not round-trip back to the original array on MySQL.

Repro

bin/run-tests.sh -i mysql:8.4 -- --filter JsonColumnTest

Why it matters

Blocks promoting the MySQL legs from non-gating to gating in ci.yml. Likely a JSON-encoding/charset or storage-format difference between MySQL and MariaDB.

Non-gating in CI today (continue-on-error).

[JJJ]

Reframed. The original "round-trip fails on MySQL" is resolved as a defined contract (f3d606a): a JSON column preserves values, types, and array/list order exactly, but object key order is not guaranteed - MySQL's native json type reorders object keys (length, then bytewise) on storage; MariaDB (json = longtext) preserves insertion order. Object member order is insignificant per RFC 8259, so cast_json() documents that and the round-trip tests compare order-insensitively (pass on both engines, no skip).

This issue now tracks the opt-in byte-exact alternative for anyone who needs object key order preserved: store JSON as longtext (like MariaDB already does) instead of the native json type. That trades native JSON validation and JSON_EXTRACT/->> functions for byte-exact fidelity, so it should be an explicit per-column opt-in, not the default. Not urgent.