chore(internal/librarian): refactor unit tests (#6557)

For unit tests of `applyDefaults`, split it into success and error
cases.

See
https://github.com/googleapis/librarian/blob/main/doc/howwewritego.md#separate-error-tests

Author: JoeWang1127

internal/librarian/library.go

@@ -15,6 +15,7 @@
 package librarian
 
 import (
+	"errors"
 	"fmt"
 	"maps"
 	"strings"
@@ -28,6 +29,10 @@ import (
 	"github.com/googleapis/librarian/internal/librarian/swift"
 )
 
+var (
+	errNoExplicitOutput = errors.New("library requires an explicit output path")
+)
+
 // fillDefaults populates empty library fields from the provided defaults.
 func fillDefaults(lib *config.Library, d *config.Default) *config.Library {
 	if d == nil {
@@ -288,7 +293,7 @@ func applyDefaults(language string, lib *config.Library, defaults *config.Defaul
 	}
 	if lib.Output == "" {
 		if isMixedLibrary(language, lib) {
-			return nil, fmt.Errorf("library %q requires an explicit output path", lib.Name)
+			return nil, fmt.Errorf("%s: %w", lib.Name, errNoExplicitOutput)
 		}
 		var apiPath string
 		if len(lib.APIs) > 0 {

internal/librarian/library_test.go

@@ -15,6 +15,7 @@
 package librarian
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -626,15 +627,14 @@ func TestFillDefaults_Go(t *testing.T) {
 	}
 }
 
-func TestPrepareLibrary(t *testing.T) {
+func TestApplyDefaults(t *testing.T) {
 	for _, test := range []struct {
 		name        string
 		language    string
 		output      string
 		rust        *config.RustCrate
 		apis        []*config.API
 		wantOutput  string
-		wantErr     bool
 		wantAPIPath string
 	}{
 		{
@@ -671,12 +671,6 @@ func TestPrepareLibrary(t *testing.T) {
 			apis:       nil,
 			wantOutput: "src/storage/test/v1",
 		},
-		{
-			name:     "veneer without output returns error",
-			language: config.LanguageRust,
-			rust:     &config.RustCrate{Modules: []*config.RustModule{{APIPath: "google/storage/v2"}}},
-			wantErr:  true,
-		},
 		{
 			name:       "veneer with explicit output succeeds",
 			language:   config.LanguageRust,
@@ -708,12 +702,6 @@ func TestPrepareLibrary(t *testing.T) {
 				Output: "src/generated",
 			}
 			got, err := applyDefaults(test.language, lib, defaults)
-			if test.wantErr {
-				if err == nil {
-					t.Fatal("expected error, got nil")
-				}
-				return
-			}
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -730,6 +718,37 @@ func TestPrepareLibrary(t *testing.T) {
 	}
 }
 
+func TestApplyDefaults_Error(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		language string
+		lib      *config.Library
+		wantErr  error
+	}{
+		{
+			name:     "veneer without output returns error",
+			language: config.LanguageRust,
+			lib: &config.Library{
+				Name: "storage",
+				Rust: &config.RustCrate{
+					Modules: []*config.RustModule{{APIPath: "google/storage/v2"}},
+				},
+			},
+			wantErr: errNoExplicitOutput,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			defaults := &config.Default{
+				Output: "src/generated",
+			}
+			_, err := applyDefaults(test.language, test.lib, defaults)
+			if !errors.Is(err, test.wantErr) {
+				t.Errorf("got error %v, want %v", err, test.wantErr)
+			}
+		})
+	}
+}
+
 func TestCanDeriveAPIPath(t *testing.T) {
 	for _, test := range []struct {
 		name     string