Stop gateway actors after slash delete

[ymote]

Summary

• wire gateway slash-command deletion into the existing session-delete actor removal channel
• notify the runtime only after SessionManager::clear succeeds
• add dispatcher regressions for named deletion and current-topic deletion notifications

Closes #1219

Validation

• CARGO_TARGET_DIR=/private/tmp/octos-1219-target cargo test -p octos-cli --features api should_notify_runtime_when -- --nocapture
• CARGO_TARGET_DIR=/private/tmp/octos-1219-target cargo test -p octos-cli --features api gateway_dispatcher::tests::should_delete -- --nocapture
• CARGO_TARGET_DIR=/private/tmp/octos-1219-target cargo clippy -p octos-cli --features api --all-targets

Notes: clippy passed with existing warnings in untouched code.

[ymote]

Closing this as a duplicate of the earlier #1220, which already targets #1219 with the same gateway /delete actor-stop fix and validation. Keeping #1220 as the active review/merge vehicle avoids duplicate closure PRs for the same issue.