Problem: #38169 used compl_used_match to determine the CompleteDone
reason, but this fires too broadly, it also changes the reason to
"accept" when the popup was shown and the user dismissed it with <Esc>
or <Space>, breaking snippet completion with autocomplete.
Solution: Instead of checking compl_used_match in, check whether the pum
was never shown (compl_match_array == NULL) in ins_compl_stop().
When a match was inserted but the pum never displayed,
set the completed word so CompleteDone fires with reason "accept".
This keeps the "discard" reason intact when the user dismisses a visible
pum without confirming.
Problem: CompleteDone fires with reason="discard" even when the candidate
text was inserted and left in the buffer, because reason was determined
solely by the terminating keycode (Ctrl-Y).
Solution: Check compl_used_match to detect whether inserted
text remains in the buffer, and set reason="accept" accordingly.
Problem: there is no way to distinguish between user's explicit
completion stop/cancel and other automated reasons.
Solution: update "cancel" reason to be set only on explicit CTRL-e, and
set intentionally vague "discard" otherwise.
Problem: `CompleteDone` currently does not specify the reason for why completion was done, which is problematic for completion plugins as they cannot know whether the event was triggered due to the completion being canceled, accepted, or for some other reason.
Solution: Add a `reason` key to `v:event`, which is set by `CompleteDone` to indicate why completion ended.
