added try..catch for reroute directive handling

Fixes withastro#9871

packages/astro/src/runtime/server/endpoint.ts

@@ -42,6 +42,27 @@ export async function renderEndpoint(
 	const response = await handler.call(mod, context);
 	// Endpoints explicitly returning 404 or 500 response status should
 	// NOT be subject to rerouting to 404.astro or 500.astro.
-	response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
-	return response;
+	return setRerouteDirective(response);
+}
+
+// This is separated into a dedicated function,
+// to allow the JavaScript runtime to optimize
+// the `renderEndpoint` implementation.
+// For more details: https://web.dev/articles/speed-v8#therefore_5
+function setRerouteDirective(response: Response): Response {
+	try {
+		response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+
+		return response;
+	} catch (_) {
+		const extendableHeaders = new Headers(response.headers);
+
+		extendableHeaders.set(REROUTE_DIRECTIVE_HEADER, 'no');
+
+		return new Response(response.body, {
+			headers: extendableHeaders,
+			status: response.status,
+			statusText: response.statusText,
+		});
+	}
 }