fix: Use HEROKU_BUILD_COMMIT with HEROKU_SLUG_COMMIT as fallback

Heroku deprecated HEROKU_SLUG_COMMIT in favor of HEROKU_BUILD_COMMIT.
Update the default release detection to check HEROKU_BUILD_COMMIT first,
keeping HEROKU_SLUG_COMMIT as a fallback for backwards compatibility with
older Heroku stacks.

Fixes GH-1218
Co-Authored-By: Claude <noreply@anthropic.com>

Author: ericapisani

util.go

@@ -41,7 +41,8 @@ func defaultRelease() (release string) {
 	// Return first non-empty environment variable known to hold release info, if any.
 	envs := []string{
 		"SENTRY_RELEASE",
-		"HEROKU_SLUG_COMMIT",
+		"HEROKU_BUILD_COMMIT",
+		"HEROKU_SLUG_COMMIT", // Deprecated, kept for backwards compatibility
 		"SOURCE_VERSION",
 		"CODEBUILD_RESOLVED_SOURCE_VERSION",
 		"CIRCLE_SHA1",

util_test.go

@@ -36,6 +36,20 @@ func TestDefaultReleaseSentryReleaseEnvvar(t *testing.T) {
 	assertEqual(t, defaultRelease(), releaseVersion)
 }
 
+func TestDefaultReleaseHerokuBuildCommitEnvvar(t *testing.T) {
+	releaseVersion := "abc123"
+	t.Setenv("HEROKU_BUILD_COMMIT", releaseVersion)
+
+	assertEqual(t, defaultRelease(), releaseVersion)
+}
+
+func TestDefaultReleaseHerokuBuildCommitPrecedenceOverSlugCommit(t *testing.T) {
+	t.Setenv("HEROKU_SLUG_COMMIT", "old-slug-value")
+	t.Setenv("HEROKU_BUILD_COMMIT", "new-build-value")
+
+	assertEqual(t, defaultRelease(), "new-build-value")
+}
+
 func TestDefaultReleaseSentryReleaseEnvvarPrecedence(t *testing.T) {
 	releaseVersion := "1.2.3"
 	t.Setenv("SOURCE_VERSION", "3.2.1")