outbound/urltest: fix gateway freeze when relay stops forwarding

[HouMinXi]

Summary

Fix transparent gateway freeze caused by URLTestGroup.urlTest() blocking indefinitely when a relay server accepts TCP but stops forwarding data.

Problem

When a relay becomes unresponsive at the application layer (TCP keepalive keeps the connection alive), three mechanisms lock simultaneously:

1. bufio.CopyConn goroutines block in Read() forever (no idle timeout on relay connections)
2. URL test probe goroutines also block in Read() waiting for HTTP response from the dead relay
3. batch.Wait() requires ALL probes to complete, so one stuck probe blocks the entire health check
4. checking atomic flag stays true, silencing all future timer-triggered health checks
5. selectedOutbound is never updated, routing all new connections to the dead relay

Confirmed by SIGQUIT goroutine dump during a live stall: 205 goroutines total, 168 blocked in CopyConn IO wait, 1 in batch.Wait() semacquire for 3 minutes. Full dump available in #4255.

Fix

common/urltest/urltest.go: Set SetReadDeadline on the dialed connection. Context cancellation does not interrupt net.Conn.Read() when the connection comes from a custom DialContext. Uses relative timeout to avoid issues with time already consumed by the dial phase.

protocol/group/urltest.go:

• Derive per-probe testCtx from batch context (was g.ctx), so batch cancellation propagates
• Wrap batch.Wait() with a hard timer (2*TCPTimeout). On timeout, proceed with available results
• Delete stale history for incomplete probes to prevent performUpdateCheck from selecting a stuck outbound

Testing

Deployed patched binary on the affected gateway (N100 iStoreOS, tproxy + urltest with 6 exit nodes). Before patch: 8 stalls in 2 hours with intervals shrinking to 57s. After patch: monitoring for stability.

Ref: #4255 (goroutine dump and full analysis), #4144 (same symptom on different deployments), #1620 (same symptom on N100 tproxy, closed as stale)