fix(arrow-go/flightsql): route QueryContext through active transaction (#692)

I used GPT-5.3-Codex to generate this patch to fix #667. I have reviewed
the outputs.

---

### Rationale for this change
`database/sql` queries executed via
`tx.QueryContext`/`tx.QueryRowContext` were not consistently bound to
the active Flight SQL transaction when no query arguments were provided.
The driver always used `c.client.Execute(...)`, which bypassed
transaction context and could not see uncommitted transactional state
(e.g., tables created inside the transaction).

### What changes are included in this PR?
- Updated `Connection.QueryContext` in the Flight SQL Go driver to use:
  - `c.txn.Execute(...)` when `c.txn` is active and valid.
  - `c.client.Execute(...)` otherwise.
- This aligns zero-argument query execution semantics with transactional
expectations in `database/sql`.

### Are these changes tested?
Yes.
- Added/used regression coverage in driver tests:
  - `TestTxQueryContextUsesTransaction`
- Validation confirms:
  - test fails when the fix is removed (cannot see tx-local table),
  - test passes when the fix is applied.

### Are there any user-facing changes?
Yes, behavior is corrected for users of the Go Flight SQL `database/sql`
driver:
- `tx.QueryContext` / `tx.QueryRowContext` (including zero-arg queries)
now execute within the active transaction as expected.
- No API or configuration changes.

Author: wjywbs

arrow/flight/flightsql/driver/driver.go

@@ -505,7 +505,15 @@ func (c *Connection) QueryContext(ctx context.Context, query string, args []driv
 		defer cancel()
 	}
 
-	info, err := c.client.Execute(execCtx, query)
+	var (
+		info *flight.FlightInfo
+		err  error
+	)
+	if c.txn != nil && c.txn.ID().IsValid() {
+		info, err = c.txn.Execute(execCtx, query)
+	} else {
+		info, err = c.client.Execute(execCtx, query)
+	}
 	if err != nil {
 		return nil, err
 	}

arrow/flight/flightsql/driver/driver_test.go

@@ -1505,6 +1505,52 @@ func (s *SqlTestSuite) TestTxRollback() {
 	wg.Wait()
 }
 
+func (s *SqlTestSuite) TestTxQueryContextUsesTransaction() {
+	t := s.T()
+
+	// Create and start the server
+	server, addr, err := s.createServer()
+	require.NoError(t, err)
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		require.NoError(s.T(), s.startServer(server))
+	}()
+	defer s.stopServer(server)
+	time.Sleep(100 * time.Millisecond)
+
+	// Configure client
+	cfg := s.Config
+	cfg.Address = addr
+	db, err := sql.Open("flightsql", cfg.DSN())
+	require.NoError(t, err)
+	defer db.Close()
+
+	ctx := context.Background()
+	tx, err := db.BeginTx(ctx, nil)
+	require.NoError(t, err)
+
+	// Create table and insert one row inside the transaction.
+	_, err = tx.ExecContext(ctx, fmt.Sprintf(s.Statements["create table"], s.TableName))
+	require.NoError(t, err)
+	_, err = tx.ExecContext(ctx, fmt.Sprintf(s.Statements["insert"], s.TableName, "inside-tx", 123))
+	require.NoError(t, err)
+
+	// This zero-argument query must execute inside the active transaction.
+	var count int
+	err = tx.QueryRowContext(ctx, fmt.Sprintf("SELECT COUNT(*) FROM %s", s.TableName)).Scan(&count)
+	require.NoError(t, err)
+	require.Equal(t, 1, count)
+
+	require.NoError(t, tx.Rollback())
+
+	// Tear-down server
+	s.stopServer(server)
+	wg.Wait()
+}
+
 func (s *SqlTestSuite) TestTxCommit() {
 	t := s.T()