[next-gen on clould] tiproxy panic with error [waitgroup/waitgroup.go:49] [panic in the recoverable goroutine] [err="runtime error: slice bounds out of range [:4] with capacity 1"]

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1、run read write workload
2、tidb auto scale out

### 2. What did you expect to see? (Required)
no panic

### 3. What did you see instead (Required)
tiproxy panic
`2025-08-19 11:50:48
[2025/08/19 03:50:48.809 +00:00] [ERROR] [main.proxy] [waitgroup/waitgroup.go:49] [panic in the recoverable goroutine] [err="runtime error: slice bounds out of range [:4] with capacity 1"] ["stack trace"="github.com/pingcap/tiproxy/lib/util/waitgroup.recoverFromErr\n\t/workspace/source/tiproxy/lib/util/waitgroup/waitgroup.go:51\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:791\nruntime.goPanicSliceAcap\n\t/usr/local/go/src/runtime/panic.go:141\ngithub.com/pingcap/tiproxy/pkg/proxy/net.ParseSSLRequestOrHandshakeResp\n\t/workspace/source/tiproxy/pkg/proxy/net/mysql.go:50\ngithub.com/pingcap/tiproxy/pkg/proxy/backend.(*Authenticator).handshakeFirstTime\n\t/workspace/source/tiproxy/pkg/proxy/backend/authenticator.go:111\ngithub.com/pingcap/tiproxy/pkg/proxy/backend.(*BackendConnManager).Connect\n\t/workspace/source/tiproxy/pkg/proxy/backend/backend_conn_mgr.go:206\ngithub.com/pingcap/tiproxy/pkg/proxy/client.(*ClientConnection).Run\n\t/workspace/source/tiproxy/pkg/proxy/client/client_conn.go:50\ngithub.com/pingcap/tiproxy/pkg/proxy.(*SQLServer).onConn\n\t/workspace/source/tiproxy/pkg/proxy/proxy.go:194\ngithub.com/pingcap/tiproxy/pkg/proxy.(*SQLServer).Run.func2.1\n\t/workspace/source/tiproxy/pkg/proxy/proxy.go:134\ngithub.com/pingcap/tiproxy/lib/util/waitgroup.(*WaitGroup).RunWithRecover.func1\n\t/workspace/source/tiproxy/lib/util/waitgroup/waitgroup.go:38"]`

### 4. What is your TiDB version? (Required)
tiproxy version
version: 8.0.11-TiDB-v9.0.0-beta.2.pre-344-ga657e4d