ci: pick highest semver Copilot LSP release instead of newest by date

The GitHub releases/latest endpoint sorts by created_at, so a backport
patch published to an older minor (e.g. 1.474.8) wins over a newer minor
already shipped (e.g. 1.487.0). Resolve the tag by parsing tag_name as
MAJOR.MINOR.PATCH and picking the highest semver, then look up the
asset URL on that specific tag. Also harden the bash steps with
set -euo pipefail and explicit empty checks.