test(pr_preflight): raise line coverage from 48% to 100%

Add 26 new mocked tests covering all major uncovered branches:
batch numbers[], ref URL/slug/bare-numeric parsing, PR-not-found,
all-bad PR with oversized commits, compareCommits failure, draft/closed
state, CHANGES_REQUESTED and pending review rows, pending CI running
row, includeLogs happy path + log-download failure + outer fetch
throw + no-failing-checks guard, localPath resolution (no-remote and
valid-remote), and behind-base count warnings. All tests are fully
deterministic with zero live network calls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: AlbinoGeek