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>

exec_test.go

@@ -0,0 +1,99 @@
+package dbx
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestInsert_ReturnsLastInsertID(t *testing.T) {
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{lastInsertID: 42}, nil
+		},
+	}
+
+	id, err := Insert(mockExecutable{}, stmt)
+	require.NoError(t, err)
+	assert.Equal(t, uint64(42), id)
+}
+
+func TestInsert_RejectsZeroID(t *testing.T) {
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{lastInsertID: 0}, nil
+		},
+	}
+
+	_, err := Insert(mockExecutable{}, stmt)
+	assert.Error(t, err)
+}
+
+func TestInsertReturning_ReturnsRow(t *testing.T) {
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, dest any) error {
+			ptr := dest.(*row)
+			ptr.ID = 7
+			return nil
+		},
+	}
+
+	result, err := InsertReturning[row](mockQueryable{}, stmt)
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	assert.Equal(t, 7, result.ID)
+}
+
+func TestUpdateAffected_ReturnsCount(t *testing.T) {
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{rowsAffected: 3}, nil
+		},
+	}
+
+	n, err := UpdateAffected(mockExecutable{}, stmt)
+	require.NoError(t, err)
+	assert.Equal(t, int64(3), n)
+}
+
+func TestDelete_Succeeds(t *testing.T) {
+	called := false
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			called = true
+			return mockResult{}, nil
+		},
+	}
+
+	err := Delete(mockExecutable{}, stmt)
+	require.NoError(t, err)
+	assert.True(t, called)
+}
+
+func TestDeleteAffected_ReturnsCount(t *testing.T) {
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{rowsAffected: 2}, nil
+		},
+	}
+
+	n, err := DeleteAffected(mockExecutable{}, stmt)
+	require.NoError(t, err)
+	assert.Equal(t, int64(2), n)
+}
+
+func TestExecContext_PropagatesError(t *testing.T) {
+	wantErr := errors.New("exec failed")
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return nil, wantErr
+		},
+	}
+
+	err := DeleteContext(context.Background(), mockExecutable{}, stmt)
+	assert.ErrorIs(t, err, wantErr)
+}