Additionally, fix a race condition where we'd still insert diagnostics
from a document or workspace pull even if the registration had been
unregistered in the time since the request was issued.

And, as a bonus: when a new pull diagnostics registration is added,
issue document pulls immediately.

This should fix regressions with basedpyright caused by
https://github.com/zed-industries/zed/pull/43703.

Release Notes:

- N/A