fix MySQL JSON append args for MariaDB

[errdefer]

Summary

Replace CAST(? AS JSON) with JSON_EXTRACT(?, '$') when appending marshaled JSON values in the MySQL sqljson dialect.

MariaDB treats JSON as an alias for LONGTEXT and does not support CAST(... AS JSON). As a result, sqljson.Append fails on MariaDB when appending non-primitive values such as objects, maps, structs, or arrays.

Using JSON_EXTRACT(?, '$') parses the argument as JSON and returns the root JSON value, preserving the intended append semantics for marshaled values while working on both MySQL and MariaDB.

Problem

Given a schema field that stores a slice of objects:

field.JSON("metadata", []map[string]string{}).
    Default([]map[string]string{}).
    StructTag(`json:"metadata"`)

Appending a non-primitive element on MariaDB:

_, err := client.Entity.UpdateOneID(id).
    AppendMetadata([]map[string]string{
        {
            "key":  "value",
            "time": time.Now().Format(time.RFC3339),
        },
    }).
    Save(ctx)

can generate an update containing CAST(? AS JSON) and fail with:

Error 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MariaDB server version for the right syntax to use near
'JSON)) END WHERE `id` = ?' at line 1

The failing SQL is generated by the MySQL sqljson dialect for non-primitive append arguments:

JSON_ARRAY_APPEND(`metadata`, '$', CAST(? AS JSON))

This works for MySQL, but not for MariaDB.

Fix

In dialect/sql/sqljson/dialect.go, update the non-primitive branch of (*mysql).appendArg to emit:

JSON_EXTRACT(?, '$')

instead of:

CAST(? AS JSON)

resulting in:

Before:

JSON_ARRAY_APPEND(`metadata`, '$', CAST(? AS JSON))

After:

JSON_ARRAY_APPEND(`metadata`, '$', JSON_EXTRACT(?, '$'))