chore(security): add gitleaks config and update ignore list for test tokens (#195)

.gitleaks.toml

@@ -0,0 +1,13 @@
+title = "gh-action-readme gitleaks configuration"
+
+[extend]
+useDefault = true
+
+# Allowlist for test files and fixtures that intentionally contain placeholder tokens.
+# These are not real secrets and are used only for testing purposes.
+[allowlist]
+description = "Test fixture files containing placeholder tokens"
+paths = [
+	'''^testutil/test_constants\.go$''',
+	'''^testdata/''',
+]