Surface extension build output on build-script failures

Co-authored-by: richardpark-msft <51494936+richardpark-msft@users.noreply.github.com>

Author: Copilot

cli/azd/extensions/microsoft.azd.extensions/internal/cmd/build.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"os/exec"
@@ -124,9 +125,13 @@ func runBuildAction(ctx context.Context, flags *buildFlags) error {
 	}
 
 	var buildWarnings []string
+	var buildFailureOutput string
 	// Flush collected validation warnings after the live TaskList canvas completes,
 	// regardless of whether a later task fails.
-	defer func() { writeCollectedWarnings(os.Stdout, buildWarnings) }()
+	defer func() {
+		writeCollectedWarnings(os.Stdout, buildWarnings)
+		writeBuildFailureOutput(os.Stderr, buildFailureOutput)
+	}()
 
 	taskList := ux.NewTaskList(nil).
 		AddTask(ux.TaskOptions{
@@ -203,10 +208,11 @@ func runBuildAction(ctx context.Context, flags *buildFlags) error {
 
 					if result, err := cmd.CombinedOutput(); err != nil {
 						flags.skipInstall = true
+						buildFailureOutput = string(result)
 
 						return ux.Error, common.NewDetailedError(
-							"Build Failed",
-							fmt.Errorf("failed to build artifacts: %s, %w", string(result), err),
+							"Build failed"+subprocessErrorTail(result),
+							fmt.Errorf("failed to build artifacts: %w", err),
 						)
 					}
 				}
@@ -420,3 +426,14 @@ func defaultBuildFlags(flags *buildFlags) {
 		flags.outputPath = "bin"
 	}
 }
+
+func writeBuildFailureOutput(writer io.Writer, buildOutput string) {
+	trimmed := strings.TrimSpace(buildOutput)
+	if trimmed == "" {
+		return
+	}
+
+	fmt.Fprintln(writer, output.WithErrorFormat("Build output:"))
+	fmt.Fprintln(writer, trimmed)
+	fmt.Fprintln(writer)
+}

cli/azd/extensions/microsoft.azd.extensions/internal/cmd/init_test.go

@@ -468,6 +468,23 @@ func TestValidationWarningSummary(t *testing.T) {
 	assert.Equal(t, "2 validation warnings", validationWarningSummary([]string{"first", "second"}))
 }
 
+func TestWriteBuildFailureOutput(t *testing.T) {
+	t.Run("writes heading and output when present", func(t *testing.T) {
+		var buf bytes.Buffer
+		writeBuildFailureOutput(&buf, "\n build failed at step 2 \n")
+
+		out := buf.String()
+		assert.Contains(t, out, "Build output:")
+		assert.Contains(t, out, "build failed at step 2")
+	})
+
+	t.Run("no output when empty", func(t *testing.T) {
+		var buf bytes.Buffer
+		writeBuildFailureOutput(&buf, "  \n\t")
+		assert.Empty(t, buf.String())
+	})
+}
+
 func TestSubprocessErrorTail(t *testing.T) {
 	tests := []struct {
 		name string