feat: add delete, context, and tx helpers

Add Delete/DeleteAffected, context-aware CRUD helpers, WithTx, ContainsCol,
and queryReturning deduplication for returning statements.

Co-authored-by: Cursor <cursoragent@cursor.com>

testhelpers_test.go

@@ -0,0 +1,72 @@
+package dbx
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/go-jet/jet/v2/qrm"
+)
+
+type mockStatement struct {
+	queryContextFn func(ctx context.Context, dest any) error
+	execContextFn  func(ctx context.Context) (sql.Result, error)
+}
+
+func (m mockStatement) Query(db qrm.Queryable, dest any) error {
+	return m.QueryContext(context.Background(), db, dest)
+}
+
+func (m mockStatement) QueryContext(ctx context.Context, db qrm.Queryable, dest any) error {
+	if m.queryContextFn != nil {
+		return m.queryContextFn(ctx, dest)
+	}
+	return nil
+}
+
+func (m mockStatement) Exec(db qrm.Executable) (sql.Result, error) {
+	return m.ExecContext(context.Background(), db)
+}
+
+func (m mockStatement) ExecContext(ctx context.Context, db qrm.Executable) (sql.Result, error) {
+	if m.execContextFn != nil {
+		return m.execContextFn(ctx)
+	}
+	return nil, nil
+}
+
+type mockQueryable struct{}
+
+func (mockQueryable) Query(string, ...any) (*sql.Rows, error) { return nil, nil }
+
+func (mockQueryable) QueryContext(context.Context, string, ...any) (*sql.Rows, error) {
+	return nil, nil
+}
+
+type mockExecutable struct{}
+
+func (mockExecutable) Exec(string, ...any) (sql.Result, error) { return nil, nil }
+
+func (mockExecutable) ExecContext(context.Context, string, ...any) (sql.Result, error) {
+	return nil, nil
+}
+
+type mockResult struct {
+	lastInsertID      int64
+	rowsAffected      int64
+	lastInsertErr     error
+	rowsAffectedErr   error
+}
+
+func (r mockResult) LastInsertId() (int64, error) {
+	return r.lastInsertID, r.lastInsertErr
+}
+
+func (r mockResult) RowsAffected() (int64, error) {
+	return r.rowsAffected, r.rowsAffectedErr
+}
+
+type row struct {
+	ID int
+}
+
+var _ Statement = mockStatement{}