fix(outbound): measure HTTP test delay on a warm connection

Since the batched prober replaced the single tester, the reported delay
came from one cold request with keep-alives disabled, so it stacked the
SOCKS handshake, proxy dial, proxy TLS, target TCP and target TLS on top
of the round-trip. Users upgrading from v2.9.4 - whose tester warmed the
connection first and timed a second request - saw several times the real
connection time.

The cold request still proves reachability and supplies the HTTP status
plus the connect/TLS/TTFB breakdown; the delay is now re-measured on a
second request over the kept-alive connection, falling back to the cold
total when the warm request fails. Bodies are drained (bounded) so the
connection returns to the pool, and the batch test asserts both requests
of a probe share one connection.

frontend/src/hooks/useXraySetting.ts

@@ -17,8 +17,9 @@ import {
 const DIRTY_POLL_MS = 1000;
 const DEFAULT_TEST_URL = 'https://www.google.com/generate_204';
 // One HTTP-mode batch request tests this many outbounds through a single
-// shared temp xray instance; chunking keeps responses bounded (~15s worst
-// case) and lands Test All results progressively.
+// shared temp xray instance; chunking keeps responses bounded (~30s worst
+// case — each probe is a cold plus a warm request) and lands Test All
+// results progressively.
 const HTTP_BATCH_CHUNK = 16;
 
 export function isUdpOutbound(outbound: unknown): boolean {