fix(internal/librariangen): fix CHANGES.md headers (#12849)

* Change ### to ## and #### to ###.
* Add link to GitHub Release.
* Preserve date.

fixes: googleapis/librarian#2045

Author: quartzmo

internal/librariangen/release/release.go

@@ -32,6 +32,8 @@ import (
 	"cloud.google.com/go/internal/postprocessor/librarian/librariangen/module"
 )
 
+var now = time.Now
+
 // Config holds the configuration for the release-init command.
 type Config struct {
 	LibrarianDir string
@@ -66,7 +68,7 @@ func Init(ctx context.Context, cfg *Config) error {
 
 		moduleDir := filepath.Join(cfg.OutputDir, lib.ID)
 		slog.Info("librariangen: processing library for release", "id", lib.ID, "version", lib.Version)
-		if err := updateChangelog(cfg, lib, time.Now().UTC()); err != nil {
+		if err := updateChangelog(cfg, lib, now().UTC()); err != nil {
 			return writeErrorResponse(cfg.LibrarianDir, fmt.Errorf("librariangen: failed to update changelog for %s: %w", lib.ID, err))
 		}
 		if err := module.GenerateInternalVersionFile(moduleDir, lib.Version); err != nil {
@@ -101,16 +103,19 @@ func updateChangelog(cfg *Config, lib *Library, t time.Time) error {
 		return fmt.Errorf("librariangen: reading changelog: %w", err)
 	}
 
-	versionString := fmt.Sprintf("### %s", lib.Version)
+	versionString := fmt.Sprintf("## [%s]", lib.Version)
 	if bytes.Contains(oldContent, []byte(versionString)) {
 		slog.Info("librariangen: changelog already up-to-date", "path", relativeChangelogPath, "version", lib.Version)
 		return nil
 	}
 
 	var newEntry bytes.Buffer
 
+	tag := fmt.Sprintf("%s/v%s", lib.ID, lib.Version)
+	encodedTag := strings.ReplaceAll(tag, "/", "%2F")
+	releaseURL := "https://github.com/googleapis/google-cloud-go/releases/tag/" + encodedTag
 	date := t.Format("2006-01-02")
-	fmt.Fprintf(&newEntry, "%s (%s)\n\n", versionString, date)
+	fmt.Fprintf(&newEntry, "## [%s](%s) (%s)\n\n", lib.Version, releaseURL, date)
 
 	changesByType := make(map[string]map[string]bool)
 	for _, change := range lib.Changes {
@@ -125,7 +130,7 @@ func updateChangelog(cfg *Config, lib *Library, t time.Time) error {
 		if len(subjects) == 0 {
 			continue
 		}
-		fmt.Fprintf(&newEntry, "#### %s\n\n", section.Section)
+		fmt.Fprintf(&newEntry, "### %s\n\n", section.Section)
 		for subj := range subjects {
 			fmt.Fprintf(&newEntry, "* %s\n", subj)
 		}

internal/librariangen/release/release_test.go

@@ -23,6 +23,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"cloud.google.com/go/internal/postprocessor/librarian/librariangen/config"
 )
@@ -102,6 +103,11 @@ func assertVersion(t *testing.T, versionGoPath, wantVersion string) {
 }
 
 func TestInit(t *testing.T) {
+	oldNow := now
+	defer func() { now = oldNow }()
+	now = func() time.Time {
+		return time.Date(2025, 9, 11, 0, 0, 0, 0, time.UTC)
+	}
 	tests := []struct {
 		name                     string
 		requestJSON              string
@@ -130,9 +136,12 @@ func TestInit(t *testing.T) {
 				"secretmanager/internal/version.go": `package internal; const Version = "1.15.0"`,
 				"internal/generated/snippets/secretmanager/apiv1/snippet_metadata.google.cloud.secretmanager.v1.json": `{"version": "1.15.0"}`,
 			},
-			wantChangelogSubstr: "### 1.16.0",
-			wantVersion:         "1.16.0",
-			wantSnippetVersion:  `"version": "1.16.0"`,
+			// This wantChangelogSubstr expectation intentionally includes two newline literals.
+			wantChangelogSubstr: `## [1.16.0](https://github.com/googleapis/google-cloud-go/releases/tag/secretmanager%2Fv1.16.0) (2025-09-11)
+
+`,
+			wantVersion:        "1.16.0",
+			wantSnippetVersion: `"version": "1.16.0"`,
 		},
 		{
 			name:                "release not triggered",
@@ -143,7 +152,7 @@ func TestInit(t *testing.T) {
 			name:        "changelog already up-to-date",
 			requestJSON: `{ "libraries": [ { "id": "secretmanager", "version": "1.16.0", "release_triggered": true, "apis": [{"path": "google/cloud/secretmanager/v1"}], "changes": [{"type": "feat", "subject": "add new GetSecret API"}] } ] }`,
 			initialRepoContent: map[string]string{
-				"secretmanager/CHANGES.md": "# Changes\n\n### 1.16.0\n- Already there.",
+				"secretmanager/CHANGES.md": "# Changes\n\n## [1.16.0](https://github.com/googleapis/google-cloud-go/releases/tag/secretmanager%2Fv1.16.0)\n- Already there.",
 				"internal/generated/snippets/secretmanager/apiv1/snippet_metadata.google.cloud.secretmanager.v1.json": `{"version": "1.15.0"}`,
 			},
 			changelogAlreadyUpToDate: true,