SelectiveChannel AddChannel失败，用户是否需要删除Channel指针？

**Describe the bug (描述bug)**
我看AddChannel函数里失败的情况，有的调用了delete sub_chan , 有的没调用。
用户侧不调用删除的话，恐怕有内存泄露；调用删除的话，可能会有double free的风险。

**To Reproduce (复现方法)**

**Expected behavior (期望行为)**

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

**Additional context/screenshots (更多上下文/截图)**

Start by reading SelectiveChannel::AddChannel and tracing every failure path involving sub_chan, comparing which paths call delete. Determine and document the ownership contract for callers, then verify the failure cases do not permit either a leak or double free.

Written by the indexing model from the issue text.