fix(onError): use Function type, await async hooks, add 4 tests

- Type: use Function for onError (consistent with all other hooks in Hooks interface)
- Async: await onError call so async hooks that reject are caught by try/catch
- Tests: 4 new tests covering sync, async, throwing, and no-error paths
- All 4 new tests pass (54 total, 1 pre-existing failure unchanged)

Author: molty3000

index.d.ts

@@ -59,7 +59,7 @@ declare namespace fastgateway {
     onRequest?: Function
     rewriteHeaders?: Function
     onResponse?: Function
-    onError?: (error: Error, req: import('http').IncomingMessage, res: import('http').ServerResponse) => void
+    onError?: Function
     rewriteRequestHeaders?: Function
     request?: {
       timeout?: number

index.js

@@ -176,7 +176,7 @@ const handler = (route, proxy, proxyHandler) => async (req, res, next) => {
     const { onError } = hooks
     if (typeof onError === 'function') {
       try {
-        onError(err, req, res)
+        await onError(err, req, res)
       } catch (_) {
         // onError hook failure must not suppress the original error
       }