fix(gonet): avoid shutdown timeout without active connections

[lmsn-kang]

What type of PR is this?

fix

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fix(gonet): 避免无活动连接时 Shutdown 错误超时

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

gonet.transServer.Shutdown currently checks the active connection count only when its fixed 100 ms ticker fires. If there are no active connections and ExitWaitTime is shorter than 100 ms, the context deadline fires first and Shutdown incorrectly returns context.DeadlineExceeded. Even with the default timeout, an idle server unnecessarily waits for the first ticker.

This change checks the connection count immediately after the existing graceful-shutdown notification. Active connections still use the existing ticker and context deadline behavior.

A regression test covers an idle server with a 10 ms exit wait time.

zh(optional):

gonet.transServer.Shutdown 原本只在固定的 100 ms ticker 触发时检查活动连接数。当没有活动连接且 ExitWaitTime 小于 100 ms 时，context 会先超时，导致 Shutdown 错误返回 context.DeadlineExceeded。本修改在原有优雅关闭通知之后立即检查一次连接数；存在活动连接时仍沿用原有轮询和超时逻辑。

Tests:

• go test ./pkg/remote/trans/gonet -count=20
• go test ./server ./server/genericserver -count=1
• go vet ./pkg/remote/trans/gonet

Manual TCP lifecycle test on Windows using a real Gonet listener and client connection:

• Idle at shutdown, conn=0, ExitWaitTime=10ms: upstream returned context deadline exceeded after 10.752 ms; this patch returned nil immediately.
• Graceful active connection, conn=1, ExitWaitTime=500ms: upstream returned nil after 100.374 ms; this patch closed the connection and returned nil immediately.
• Intentionally stuck connection, conn=1, ExitWaitTime=30ms: both versions correctly returned context deadline exceeded after about 30 ms.

(Optional) Which issue(s) this PR fixes:

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(optional) The PR that updates user documentation:

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