chore: fix typos, remove stale comment, and update codecov-action to v4

- Remove misleading "For Debuging" comment in sys_session.go
- Fix "offical" → "official" typo in handlers_session.go
- Update codecov-action@v3 → @v4 in CI workflow
- Consolidate technical-debt.md with completed items and updated TOC

server/channelserver/sys_session.go

@@ -70,7 +70,6 @@ type Session struct {
 	// Contains the mail list that maps accumulated indexes to mail IDs
 	mailList []int
 
-	// For Debuging
 	Name     string
 	closed   atomic.Bool
 	ackStart map[uint32]time.Time