Remove usage of `r_task()`

Currently there are many places where we evaluate R code / access the R API through the use of r_task(), which hooks into R's polled events. This makes it easy to implement analytic features but has a number of downsides:

• The R event polls might be slowed down of stuck if native code running in the R session doesn't check for interrupts sufficiently often.

• While R polls at specific times such as checks for interrupts, in practice we should consider polled events to be preemptive. This is unsafe because we might accidentally call a non-rentrant function, for instance we might force a promise that causes a loadNamespace(), which is not reentrant, while it is already running. It is difficult to manage these risks as the project grows and many features are added, querying R all the time.

So we should strive to reduce concurrent evaluations to the bare minimum:

• https://github.com/posit-dev/positron/issues/2284
• https://github.com/posit-dev/ark/issues/689
• https://github.com/posit-dev/positron/issues/2321
  https://github.com/posit-dev/ark/issues/1095

The issue does not name files or tests; start by locating the usages and implementation of r_task() in the ark repository, then review linked issues #689, #2321, and #1095 for scope. Done means the remaining concurrent R evaluations are reduced to the intended minimum and the referenced work items are addressed.