refactor(outbound): probe via xray burstObservatory instead of SOCKS round-trip

Replace the HTTP-mode outbound test that spun up a SOCKS inbound and
ran an httptrace'd request from the Go client with a probe-only xray
config: burstObservatory probes the target outbound directly and the
result is read from xray's /debug/vars metrics endpoint.

The probe lives inside xray, so the measured delay and failure
reasons reflect what xray itself sees over the real proxy chain.
Drops the DNS/Connect/TLS/TTFB breakdown (and statusCode) since the
observatory snapshot only exposes total delay; the frontend popover
is updated accordingly.

frontend/src/schemas/xray.ts

@@ -54,11 +54,6 @@ export const OutboundTestResultSchema = z.object({
   delay: z.number().optional(),
   error: z.string().optional(),
   mode: z.string().optional(),
-  ttfbMs: z.number().optional(),
-  tlsMs: z.number().optional(),
-  connectMs: z.number().optional(),
-  dnsMs: z.number().optional(),
-  statusCode: z.number().optional(),
   endpoints: z
     .array(
       z.object({