Fix govulncheck failure by removing lib/pq from default builds

govulncheck fails on seven unpatched lib/pq vulnerabilities
(GO-2026-6166 through GO-2026-6171). All of them are reported with
'Fixed in: N/A', so bumping the dependency cannot fix the check.

The sqlc binary itself never uses lib/pq; the module only linked it in
two places that govulncheck's default (untagged) scan could see:

- internal/sqltest/postgres.go registered the lib/pq driver, but its
  helpers (PostgreSQL, CreatePostgreSQLDatabase) have no callers left,
  so delete the file and move the id() helper to mysql.go, which still
  uses it.

- examples/ondeck/postgresql generated code imports lib/pq for
  pq.Array. Its tests are already build-tagged 'examples', so set
  build_tags: examples for the package in sqlc.json and regenerate,
  putting the generated files behind the same tag as the tests that
  exercise them.

With no lib/pq import left in the default build, govulncheck reports
zero called vulnerabilities; lib/pq remains a module requirement for
the examples-tagged tests, which is informational only.

Verified with govulncheck ./... (0 findings), go build/vet with and
without the examples tag, and the example test suites against live
PostgreSQL and MySQL.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011mXrXXbgLobj5jpsyzJqB8

Author: claude

examples/ondeck/postgresql/city.sql.go

@@ -20,6 +20,7 @@
       "rules": [
         "sqlc/db-prepare"
       ],
+      "build_tags": "examples",
       "emit_json_tags": true,
       "emit_prepared_queries": true,
       "emit_interface": true

examples/ondeck/postgresql/db.go

@@ -3,15 +3,31 @@ package sqltest
 import (
 	"database/sql"
 	"fmt"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 
 	_ "github.com/go-sql-driver/mysql"
 
 	"github.com/sqlc-dev/sqlc/internal/sql/sqlpath"
 )
 
+func init() {
+	rand.Seed(time.Now().UnixNano())
+}
+
+var letterRunes = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+
+func id() string {
+	b := make([]rune, 10)
+	for i := range b {
+		b[i] = letterRunes[rand.Intn(len(letterRunes))]
+	}
+	return string(b)
+}
+
 func MySQL(t *testing.T, migrations []string) (*sql.DB, func()) {
 	// For each test, pick a new database name at random.
 	name := "sqltest_mysql_" + id()