remove redandunt fallback logic

Author: zhumin8

internal/librarian/java/generate_test.go

@@ -677,6 +677,7 @@ func TestGenerate_Logic(t *testing.T) {
 		Libraries: []*config.Library{
 			library,
 			{Name: rootLibrary, Version: "1.2.3"},
+			{Name: parentPOM, Version: "1.2.3"},
 		},
 	}
 	// Setup mandatory files for postProcessAPI and syncPOMs
@@ -753,6 +754,7 @@ func TestGenerate_ProtoExclusion(t *testing.T) {
 		Libraries: []*config.Library{
 			library,
 			{Name: rootLibrary, Version: "1.2.3"},
+			{Name: parentPOM, Version: "1.2.3"},
 		},
 	}
 	err := Generate(t.Context(), cfg, library, &sources.Sources{Googleapis: googleapisDir})

internal/librarian/java/pom.go

@@ -526,17 +526,13 @@ func findMonorepoVersion(cfg *config.Config) (string, error) {
 	return "", errMonorepoVersion
 }
 
+// TODO(https://github.com/googleapis/librarian/issues/6411):
+// Simplify logic here and check at validate step.
 func findParentPOMVersion(cfg *config.Config) (string, error) {
 	for _, lib := range cfg.Libraries {
 		if lib.Name == parentPOM {
 			return lib.Version, nil
 		}
 	}
-	// Fallback to monorepoVersion if google-cloud-pom-parent is missing (e.g. in tests)
-	for _, lib := range cfg.Libraries {
-		if lib.Name == rootLibrary {
-			return lib.Version, nil
-		}
-	}
 	return "", errParentVersion
 }

internal/librarian/java/postgenerate_test.go

@@ -41,6 +41,7 @@ func TestPostGenerate(t *testing.T) {
 		Language: "java",
 		Libraries: []*config.Library{
 			{Name: rootLibrary, Version: "1.2.3"},
+			{Name: parentPOM, Version: "1.2.3"},
 			{Name: "analytics-admin", Version: "0.98.0"},
 			{Name: "area120-tables", Version: "0.92.0"},
 			{Name: "aiplatform", Version: "3.89.0"},
@@ -196,6 +197,7 @@ func TestPostGenerate_SearchError(t *testing.T) {
 	cfg := &config.Config{
 		Libraries: []*config.Library{
 			{Name: rootLibrary, Version: "1.2.3"},
+			{Name: parentPOM, Version: "1.2.3"},
 		},
 	}
 	err := PostGenerate(t.Context(), tmpDir, cfg, nil)
@@ -215,6 +217,7 @@ func TestPostGenerate_Error(t *testing.T) {
 	cfg := &config.Config{
 		Libraries: []*config.Library{
 			{Name: rootLibrary, Version: "1.2.3"},
+			{Name: parentPOM, Version: "1.2.3"},
 		},
 	}
 	err := PostGenerate(t.Context(), tmpDir, cfg, nil)

internal/librarian/java/postprocess_test.go

@@ -520,6 +520,7 @@ func TestPostProcessLibrary(t *testing.T) {
 	defaultCfg := &config.Config{
 		Libraries: []*config.Library{
 			{Name: rootLibrary, Version: "1.0.0"},
+			{Name: parentPOM, Version: "1.0.0"},
 		},
 		Default: &config.Default{
 			Java: &config.JavaModule{
@@ -621,6 +622,7 @@ func TestPostProcessLibrary_ErrorCase(t *testing.T) {
 	defaultCfg := &config.Config{
 		Libraries: []*config.Library{
 			{Name: rootLibrary, Version: "1.0.0"},
+			{Name: parentPOM, Version: "1.0.0"},
 		},
 		Default: &config.Default{
 			Java: &config.JavaModule{