Add ready notification to ai context feature startup, prevent missing /include menu

There is currently a race condition when launching the extension.

When the Duo Chat webview initialises, it requests the available context categories. This determines which categories are available within the /include menu (or if /include is available at all).

Sometimes, this request is sent before the language server has finished starting up, causing empty categories array to be returned. This means the /include slash command is not available. Closing Duo Chat and re-openeing after the language server has started up fixes the issue.

We should consider a notification from the language server when context features are all ready to go, at which point Duo Chat can request its categories.