Composer clear-topic and new-context tools removed

What changed

The v2 composer no longer includes the clear-topic tool button or the new-context tool button. The composer-level clear shortcut is also not carried forward as part of this input surface.

Why this matters to the user

Users who previously used the input box toolbar to clear the current topic or start a new context will no longer see those actions in the composer in v2.

What the user should do

Use topic-level controls where available, or start a new topic when a clean conversation context is needed.

Notes for release manager

This change removes the legacy Inputbar tool registrations for clear_topic and new_context from the composer tool system. The underlying topic management surfaces may still provide separate topic-level actions.