fix(internal/librarian): generate should skip formatting if library is skipped (#3325)

Skip formatting if library generate code is skipped per config,
otherwise it panics trying to format on a nil library passed in.


Local test to generate a skipped module now completes with no err:
```
╰─$ librarian generate common  
⊘ Skipping common (skip_generate is set)
```

Fix #3323

Author: zhumin8

internal/librarian/generate.go

@@ -94,6 +94,10 @@ func runGenerate(ctx context.Context, all bool, libraryName string) error {
 	if err != nil {
 		return err
 	}
+	if lib == nil {
+		// Skip formatting if generation skipped.
+		return nil
+	}
 	return formatLibrary(ctx, cfg.Language, lib)
 }
 
@@ -113,6 +117,10 @@ func generateAll(ctx context.Context, cfg *config.Config) error {
 		if err != nil {
 			return err
 		}
+		if lib == nil {
+			// Skip formatting if generation skipped.
+			continue
+		}
 		if err := formatLibrary(ctx, cfg.Language, lib); err != nil {
 			return err
 		}
@@ -315,6 +323,9 @@ func generate(ctx context.Context, language string, library *config.Library, sou
 func formatLibrary(ctx context.Context, language string, library *config.Library) error {
 	switch language {
 	case "testhelper":
+		// Access library.Name to simulate a real formatter and trigger
+		// a panic if library is nil.
+		_ = library.Name
 		return nil
 	case "rust":
 		return rust.Format(ctx, library)

internal/librarian/generate_test.go

@@ -32,37 +32,20 @@ func TestGenerateCommand(t *testing.T) {
 		lib1Output = "output1"
 		lib2       = "library-two"
 		lib2Output = "output2"
+		lib3       = "library-three"
+		lib3Output = "output3"
 	)
-	tempDir := t.TempDir()
-	t.Chdir(tempDir)
-	googleapisDir := createGoogleapisServiceConfigs(t, tempDir, map[string]string{
+	baseTempDir := t.TempDir()
+	googleapisDir := createGoogleapisServiceConfigs(t, baseTempDir, map[string]string{
 		"google/cloud/speech/v1":       "speech_v1.yaml",
 		"grafeas/v1":                   "grafeas_v1.yaml",
 		"google/cloud/texttospeech/v1": "texttospeech_v1.yaml",
 	})
-	configPath := filepath.Join(tempDir, librarianConfigPath)
-	configContent := fmt.Sprintf(`language: testhelper
-sources:
-  googleapis:
-    dir: %s
-libraries:
-  - name: %s
-    output: %s
-    channels:
-      - path: google/cloud/speech/v1
-      - path: grafeas/v1
-  - name: %s
-    output: %s
-    channels:
-      - path: google/cloud/texttospeech/v1
-`, googleapisDir, lib1, lib1Output, lib2, lib2Output)
-	if err := os.WriteFile(configPath, []byte(configContent), 0644); err != nil {
-		t.Fatal(err)
-	}
 
 	allLibraries := map[string]string{
 		lib1: lib1Output,
 		lib2: lib2Output,
+		lib3: lib3Output,
 	}
 
 	for _, test := range []struct {
@@ -91,8 +74,39 @@ libraries:
 			args: []string{"librarian", "generate", "--all"},
 			want: []string{lib1, lib2},
 		},
+		{
+			name: "skip generate",
+			args: []string{"librarian", "generate", lib3},
+			want: []string{},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
+			tempDir := t.TempDir()
+			t.Chdir(tempDir)
+			configContent := fmt.Sprintf(`language: testhelper
+sources:
+  googleapis:
+    dir: %s
+libraries:
+  - name: %s
+    output: %s
+    channels:
+      - path: google/cloud/speech/v1
+      - path: grafeas/v1
+  - name: %s
+    output: %s
+    channels:
+      - path: google/cloud/texttospeech/v1
+  - name: %s
+    output: %s
+    skip_generate: true
+    channels:
+      - path: google/cloud/speech/v1
+`, googleapisDir, lib1, lib1Output, lib2, lib2Output, lib3, lib3Output)
+			if err := os.WriteFile(filepath.Join(tempDir, librarianConfigPath), []byte(configContent), 0644); err != nil {
+				t.Fatal(err)
+			}
+
 			err := Run(t.Context(), test.args...)
 			if test.wantErr != nil {
 				if !errors.Is(err, test.wantErr) {