Fix MySQL test cleanup by using DELETE instead of TRUNCATE

The original approach of disabling foreign key checks before TRUNCATE
was unreliable due to MySQL's session-based FOREIGN_KEY_CHECKS setting
and Sequel's connection pooling. Each db.run() call can get a different
connection from the pool, so SET FOREIGN_KEY_CHECKS=0 executed on one
connection doesn't affect TRUNCATE statements executed on other connections.

Solution: Use DELETE FROM instead of TRUNCATE for MySQL test cleanup.
While slower, DELETE respects foreign key constraints by deleting in
the correct order, and doesn't require disabling FK checks.

PostgreSQL continues to use TRUNCATE with CASCADE, which handles
foreign keys correctly in a single statement.

This fixes the 208 test failures seen in CI with MySQL 8.2.

Author: joyvuu-dave

spec/support/table_truncator.rb

@@ -9,17 +9,20 @@ 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
-        tables.each do |table|
-          db.run("TRUNCATE TABLE #{table};")
-        end
+      end
+    when :mysql
+      # Use DELETE instead of TRUNCATE for MySQL to avoid FK constraint issues
+      # with connection pooling. TRUNCATE requires SET FOREIGN_KEY_CHECKS=0,
+      # which is session-specific and doesn't work reliably across pooled connections.
+      tables.each do |table|
+        db.run("DELETE FROM #{table};")
       end
     end
   end