[FIX] fix nil listener cleanup callbacks

[222twotwotwo]

What this PR does:

• fix the nil listener guard in handlePackage error cleanup by requiring both session and listener before calling OnError
• guard OnClose when the listener is nil so the same cleanup path does not panic one line later
• add a regression test for the nil-listener error cleanup path

Which issue(s) this PR fixes:

Fixes #132

Special notes for your reviewer:

The cleanup path used s != nil || s.listener != nil, so a non-nil session with a nil listener still attempted to call s.listener.OnError(...) and could panic while handling an existing read/decode error.

Does this PR introduce a user-facing change?:

NONE

[222twotwotwo]

ci错误貌似不是本次pr引入，可能是由于Makefile 里的 format 步骤现在会失败：

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -category=efaceany -fix -test ./...

原因是 modernize@latest 的 CLI 已经移除了 -category 参数；