Add cmdio test helpers for output capture (#4334)

## Changes

Adds two new test helper functions to reduce boilerplate:
- NewTestContextWithStdout: Captures stdout, discards stderr
- NewTestContextWithStderr: Captures stderr, discards stdout

Migrates existing tests in bundle/render and cmd/pipelines to use the
new helpers.

## Why

Reduces number of lines required for test setup.

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: pietern

bundle/render/render_text_output_test.go

@@ -13,7 +13,6 @@ import (
 	"github.com/databricks/cli/libs/cmdio"
 	"github.com/databricks/cli/libs/diag"
 	"github.com/databricks/cli/libs/dyn"
-	"github.com/databricks/cli/libs/flags"
 	"github.com/databricks/cli/libs/logdiag"
 	"github.com/databricks/databricks-sdk-go/service/catalog"
 	"github.com/databricks/databricks-sdk-go/service/iam"
@@ -277,15 +276,12 @@ func TestRenderDiagnostics(t *testing.T) {
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			writer := &bytes.Buffer{}
-			ctx := context.Background()
-			cmdIO := cmdio.NewIO(ctx, flags.OutputText, nil, io.Discard, writer, "", "")
-			ctx = cmdio.InContext(ctx, cmdIO)
+			ctx, stderr := cmdio.NewTestContextWithStderr(context.Background())
 
 			err := cmdio.RenderDiagnostics(ctx, tc.diags)
 			require.NoError(t, err)
 
-			assert.Equal(t, tc.expected, writer.String())
+			assert.Equal(t, tc.expected, stderr.String())
 		})
 	}
 }

cmd/pipelines/run_test.go

@@ -1,13 +1,11 @@
 package pipelines
 
 import (
-	"bytes"
 	"context"
 	"testing"
 	"time"
 
 	"github.com/databricks/cli/libs/cmdio"
-	"github.com/databricks/cli/libs/flags"
 	"github.com/databricks/databricks-sdk-go/service/pipelines"
 	"github.com/stretchr/testify/assert"
 )
@@ -146,15 +144,11 @@ Pipeline configurations for this update:
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			var buf bytes.Buffer
-
-			ctx := context.Background()
-			cmdIO := cmdio.NewIO(ctx, flags.OutputText, nil, &buf, &buf, "", "")
-			ctx = cmdio.InContext(ctx, cmdIO)
+			ctx, stdout := cmdio.NewTestContextWithStdout(context.Background())
 
 			err := displayPipelineUpdate(ctx, tt.update, tt.pipelineID, tt.events)
 			assert.NoError(t, err)
-			assert.Equal(t, tt.expected, buf.String())
+			assert.Equal(t, tt.expected, stdout.String())
 		})
 	}
 }
@@ -376,15 +370,11 @@ RUNNING                   750ms
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			var buf bytes.Buffer
-
-			ctx := context.Background()
-			cmdIO := cmdio.NewIO(ctx, flags.OutputText, nil, &buf, &buf, "", "")
-			ctx = cmdio.InContext(ctx, cmdIO)
+			ctx, stdout := cmdio.NewTestContextWithStdout(context.Background())
 
 			err := displayProgressEventsDurations(ctx, tt.events)
 			assert.NoError(t, err)
-			assert.Equal(t, tt.expected, buf.String())
+			assert.Equal(t, tt.expected, stdout.String())
 		})
 	}
 }

libs/cmdio/testing.go

@@ -2,8 +2,11 @@ package cmdio
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"io"
+
+	"github.com/databricks/cli/libs/flags"
 )
 
 type Test struct {
@@ -62,3 +65,19 @@ func SetupTest(ctx context.Context, opts TestOptions) (context.Context, *Test) {
 		Stderr: bufio.NewReader(rerr),
 	}
 }
+
+// NewTestContextWithStdout creates a cmdio context that captures stdout output.
+// Stderr is discarded. Use this for testing data output and results.
+func NewTestContextWithStdout(ctx context.Context) (context.Context, *bytes.Buffer) {
+	stdout := &bytes.Buffer{}
+	cmdIO := NewIO(ctx, flags.OutputText, nil, stdout, io.Discard, "", "")
+	return InContext(ctx, cmdIO), stdout
+}
+
+// NewTestContextWithStderr creates a cmdio context that captures stderr output.
+// Stdout is discarded. Use this for testing diagnostics, logs, and error messages.
+func NewTestContextWithStderr(ctx context.Context) (context.Context, *bytes.Buffer) {
+	stderr := &bytes.Buffer{}
+	cmdIO := NewIO(ctx, flags.OutputText, nil, io.Discard, stderr, "", "")
+	return InContext(ctx, cmdIO), stderr
+}

libs/cmdio/testing_test.go

@@ -0,0 +1,31 @@
+package cmdio_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/databricks/cli/libs/cmdio"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewTestContextWithStdout(t *testing.T) {
+	ctx, stdout := cmdio.NewTestContextWithStdout(context.Background())
+
+	// Render writes to stdout
+	data := map[string]string{"message": "test output"}
+	err := cmdio.Render(ctx, data)
+	require.NoError(t, err)
+
+	assert.Contains(t, stdout.String(), "test output")
+}
+
+func TestNewTestContextWithStderr(t *testing.T) {
+	ctx, stderr := cmdio.NewTestContextWithStderr(context.Background())
+
+	require.NotPanics(t, func() {
+		cmdio.LogString(ctx, "test message")
+	})
+
+	assert.Contains(t, stderr.String(), "test message")
+}