feat(tx): add WithTxValue for transactional return values

WithTxValue runs a callback inside a transaction and returns its result,
with rollback on error. Includes tests and documentation updates.

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

tx_test.go

@@ -0,0 +1,40 @@
+package dbx
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWithTxValue_ReturnsResult(t *testing.T) {
+	db := openTestDB(t)
+
+	result, err := WithTxValue(context.Background(), db, func(tx QueryExec) (int, error) {
+		return 42, nil
+	})
+	require.NoError(t, err)
+	assert.Equal(t, 42, result)
+}
+
+func TestWithTxValue_RollbackOnError(t *testing.T) {
+	db := openTestDB(t)
+	fail := errors.New("fail")
+
+	_, err := WithTxValue(context.Background(), db, func(tx QueryExec) (int, error) {
+		return 0, fail
+	})
+	assert.ErrorIs(t, err, fail)
+}
+
+func TestWithTxValue_CancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, err := WithTxValue(ctx, openTestDB(t), func(tx QueryExec) (int, error) {
+		return 1, nil
+	})
+	assert.ErrorIs(t, err, context.Canceled)
+}