editor: Do not show inline completion if snippet is active (#20300)

This avoids inline completions being shown (and overriding `<tab>`
behavior) when a snippet is active and the user wants to go through
snippet placeholders with `<tab>`.

Easy to reproduce:

Open a Rust file and use the `tfn` snippet to produce a test function.
Delete the placeholder. Without the change here, the inline provider
would suggest a function name. If you `<tab>`, you accept it, but then
you can't `<tab>` into the function body.

With this change the inline completions are deactivated as long as a
snippet is active.

Closes #19484 

Release Notes:

- Fixed inline completions (Copilot, Supermaven, ...) taking over when a
snippet completion was active. That resulted in `tab` not working to
jump to the next placeholder in the snippet.