Use the same `InlineAssist` action between both `assistant` and `assistant2` (#22126)

This PR makes it so `assistant` and `assistant2` both use the same
action for inline assist (`zed_actions::InlineAssist`).

This makes it so the keybindings to deploy the inline assist seamlessly
swap based on the feature flag without needing to rebind them.

One minor caveat: if you're using `assistant2` the action name in the
command palette will be `assistant: inline assist`.

Release Notes:

- N/A