chore(tests): Add database cleanup functions (googleapis#1528)

## Description

---
This change adds helper functions to cleanup the test databases
(PostgreSQL, MySQL, and MSSQL) by dropping any orphan tables from
previous test runs. These functions can be called at the beginning of
each test run.

## PR Checklist

---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

🛠️ Fixes googleapis#1304

---------

Co-authored-by: Wenxin Du <117315983+duwenxin99@users.noreply.github.com>

Author: Myst9

tests/common.go

@@ -21,6 +21,7 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/goccy/go-yaml"
@@ -804,3 +805,98 @@ func TestCloudSQLMySQL_IPTypeParsingFromYAML(t *testing.T) {
 		})
 	}
 }
+
+// Finds and drops all tables in a postgres database.
+func CleanupPostgresTables(t *testing.T, ctx context.Context, pool *pgxpool.Pool) {
+	query := `
+	SELECT table_name FROM information_schema.tables 
+	WHERE table_schema = 'public' AND table_type = 'BASE TABLE';`
+
+	rows, err := pool.Query(ctx, query)
+	if err != nil {
+		t.Fatalf("Failed to query for all tables in 'public' schema: %v", err)
+	}
+	defer rows.Close()
+
+	var tablesToDrop []string
+	for rows.Next() {
+		var tableName string
+		if err := rows.Scan(&tableName); err != nil {
+			t.Errorf("Failed to scan table name: %v", err)
+			continue
+		}
+		tablesToDrop = append(tablesToDrop, fmt.Sprintf("public.%q", tableName))
+	}
+
+	if len(tablesToDrop) == 0 {
+		return
+	}
+
+	dropQuery := fmt.Sprintf("DROP TABLE IF EXISTS %s CASCADE;", strings.Join(tablesToDrop, ", "))
+	
+	if _, err := pool.Exec(ctx, dropQuery); err != nil {
+		t.Fatalf("Failed to drop all tables in 'public' schema: %v", err)
+	}
+}
+
+// Finds and drops all tables in a mysql database.
+func CleanupMySQLTables(t *testing.T, ctx context.Context, pool *sql.DB) {
+	query := `
+	SELECT table_name FROM information_schema.tables 
+	WHERE table_schema = DATABASE() AND table_type = 'BASE TABLE';`
+
+	rows, err := pool.QueryContext(ctx, query)
+	if err != nil {
+		t.Fatalf("Failed to query for all MySQL tables: %v", err)
+	}
+	defer rows.Close()
+
+	var tablesToDrop []string
+	for rows.Next() {
+		var tableName string
+		if err := rows.Scan(&tableName); err != nil {
+			t.Errorf("Failed to scan MySQL table name: %v", err)
+			continue
+		}
+		tablesToDrop = append(tablesToDrop, fmt.Sprintf("`%s`", tableName))
+	}
+
+	if len(tablesToDrop) == 0 {
+		return
+	}
+
+	// Disable foreign key checks, drop all tables and re-enable
+	if _, err := pool.ExecContext(ctx, "SET FOREIGN_KEY_CHECKS = 0;"); err != nil {
+		t.Fatalf("Failed to disable MySQL foreign key checks: %v", err)
+	}
+
+	dropQuery := fmt.Sprintf("DROP TABLE IF EXISTS %s;", strings.Join(tablesToDrop, ", "))
+	
+	if _, err := pool.ExecContext(ctx, dropQuery); err != nil {
+		// Try to re-enable checks even if drop fails
+		if _, err := pool.ExecContext(ctx, "SET FOREIGN_KEY_CHECKS = 1;"); err != nil {
+			t.Logf("Also failed to re-enable foreign key checks: %v", err)
+		}
+		t.Fatalf("Failed to drop all MySQL tables: %v", err)
+	}
+
+	// Re-enable foreign key checks
+	if _, err := pool.ExecContext(ctx, "SET FOREIGN_KEY_CHECKS = 1;"); err != nil {
+		t.Fatalf("Failed to re-enable MySQL foreign key checks: %v", err)
+	}
+}
+
+// Finds and drops all tables in an mssql database.
+func CleanupMSSQLTables(t *testing.T, ctx context.Context, pool *sql.DB) {
+	disableConstraintsCmd := "EXEC sp_MSforeachtable 'ALTER TABLE ? NOCHECK CONSTRAINT ALL'"
+	if _, err := pool.ExecContext(ctx, disableConstraintsCmd); err != nil {
+		t.Fatalf("Failed to disable MSSQL constraints: %v", err)
+	}
+
+	// drop 'U' (User Tables)
+	dropTablesCmd := "EXEC sp_MSforeachtable 'DROP TABLE ?', @whereand = 'AND O.Type = ''U'''"
+	if _, err := pool.ExecContext(ctx, dropTablesCmd); err != nil {
+		t.Fatalf("Failed to drop all MSSQL tables: %v", err)
+	}
+
+}