Fix MySQL test cleanup using db.synchronize for same connection

The root cause of the FK constraint violations was that MySQL's
SET FOREIGN_KEY_CHECKS is session-specific (per-connection). When using
Sequel's connection pool, each db.run() call can get a different connection,
so SET FOREIGN_KEY_CHECKS=0 executed on one connection doesn't affect
TRUNCATE statements executed on other connections.

Solution: Use db.synchronize to hold a single connection from the pool
and execute all operations (SET FK=0, TRUNCATE, SET FK=1) on that same
connection using conn.query() instead of db.run().

This ensures the FK checks are properly disabled for all TRUNCATE operations
while they execute, then re-enabled afterward, all on the same connection.

PostgreSQL continues to use TRUNCATE with CASCADE, which doesn't require
this workaround as it handles foreign keys correctly in a single statement.

This fixes thousands of FK constraint violations in MySQL 8.2 tests.

Author: joyvuu-dave

spec/support/table_truncator.rb

@@ -9,17 +9,25 @@ def self.isolated_tables(db)
   end
 
   def truncate_tables
-    referential_integrity = ReferentialIntegrity.new(db)
-    referential_integrity.without do
-      case db.database_type
-      when :postgres
+    case db.database_type
+    when :postgres
+      referential_integrity = ReferentialIntegrity.new(db)
+      referential_integrity.without do
         tables.each do |table|
           db.run("TRUNCATE TABLE #{table} RESTART IDENTITY CASCADE;")
         end
-      when :mysql
+      end
+    when :mysql
+      # Use db.synchronize to ensure SET FOREIGN_KEY_CHECKS and all TRUNCATE
+      # statements run on the SAME connection from the pool. Without this,
+      # each db.run() can get a different connection, and SET FOREIGN_KEY_CHECKS=0
+      # (which is session-specific) won't apply to the TRUNCATE statements.
+      db.synchronize do |conn|
+        conn.query('SET FOREIGN_KEY_CHECKS = 0')
         tables.each do |table|
-          db.run("TRUNCATE TABLE #{table};")
+          conn.query("TRUNCATE TABLE #{table}")
         end
+        conn.query('SET FOREIGN_KEY_CHECKS = 1')
       end
     end
   end