fix(channels): stop stale typing loops on overwrite

[Alix-007]

Summary

• stop the previous typing loop immediately when the same channel/chat registers a replacement stop function
• keep the replacement stop callback active until the response is sent
• add a regression test for repeated typing registration on the same chat

Testing

• go test ./pkg/channels -run ''Test(PreSend_TypingStopCalled|RecordTypingStop|TypingStopJanitorEviction)''

Fixes #1323
Fixes #1212