restore TestPostProcessAPI_AlternateHeaders and remove duplicate test case

Author: sofisl

internal/librarian/java/postprocess_test.go

@@ -470,6 +470,43 @@ func TestPostProcessAPI_SkipHeaders(t *testing.T) {
 	}
 }
 
+func TestPostProcessAPI_AlternateHeaders(t *testing.T) {
+	t.Parallel()
+	outdir := t.TempDir()
+	apiBase := "v1"
+	gRPCDir := filepath.Join(outdir, apiBase, "grpc")
+	if err := os.MkdirAll(gRPCDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	grpcFile := filepath.Join(gRPCDir, "GRPCFile.java")
+	if err := os.WriteFile(grpcFile, []byte("package com.test;"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	params := postProcessParams{
+		outDir:  outdir,
+		apiBase: apiBase,
+		library: &config.Library{Java: &config.JavaModule{}},
+		javaAPI: &config.JavaAPI{},
+	}
+	altHeader := "/* Alternate */\n"
+	headerFile := filepath.Join(outdir, "header.txt")
+	if err := os.WriteFile(headerFile, []byte(altHeader), 0644); err != nil {
+		t.Fatal(err)
+	}
+	params.library.Java.AlternateHeaders = "header.txt"
+	if err := addHeaders(params, []string{gRPCDir}); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(grpcFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantHeader := "/* Alternate */"
+	if !bytes.HasPrefix(got, []byte(wantHeader)) {
+		t.Errorf("mismatch got = %q, want %q", got, wantHeader)
+	}
+}
+
 func TestCopyProtos_Success(t *testing.T) {
 	t.Parallel()
 	destDir := t.TempDir()
@@ -820,19 +857,6 @@ func TestAddMissingHeaders(t *testing.T) {
 			content:     "some text",
 			wantContent: "some text",
 		},
-		{
-			name: "alternate header",
-			params: postProcessParams{
-				library: &config.Library{
-					Java: &config.JavaModule{
-						AlternateHeaders: "header.txt",
-					},
-				},
-			},
-			filename:    "AlternateHeader.java",
-			content:     "package com.example;",
-			wantContent: "/* Alternate Header */\npackage com.example;",
-		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()