This function queries an LLM to write unit tests for selected R code. To do so, it:
Initializes a ensurer(): an ellmer Chat() that knows how
to write testthat unit tests.
Reads the contents of the active .R file as well as the current selection.
Opens a corresponding test file (creating it if need be).
Asks the LLM to write unit tests for the current selection, using the
contents of the active .R file as context.
Streams the response into the corresponding test file.
ensure_that()TRUE, invisibly.