[Backport release-1.6] fix(e2e): disable guest fsync on ephemeral CI VM disks - #4431

The audit started from labels and dropped every backport PR whose
original was not a candidate for the branch. On release-1.6 that hid
nine hand backports of unlabelled main PRs (#4431 to #4435, #4437,
#4438, #4456 and #4475), and it had no way to notice two open backport
PRs for the same originals: #4421 sat open next to #4456 for #3938 and
#4280, and nothing reported it.

Read the PRs on the branch from the side of the originals they claim as
well, and report two more sections per branch.

UNLABELLED lists each original claimed by backport PRs on the branch
that is not a candidate for it, with every backport PR claiming it and
its state, and says whether they backported it, only claim it with a PR
still open, or were all closed. It never moves the exit code: the gate
answers whether everything labelled landed, and an unlabelled backport
can only add to a branch, never leave a labelled change off it.

DUPLICATE lists each original claimed by two open backport PRs, or by
an open one after another already merged, labelled or not. Both make
the audit exit 1. One of the PRs is redundant, or the open one is the
rest of a split backport; either way someone has to decide before the
cut, and no verdict can say so, since a verdict settles on the first
merged backport or reports the first open one as pending. A closed PR
next to an open one is how a conflicting bot backport gets redone by
hand and is not flagged.

The titles of originals that no listing carries come from one GraphQL
request for the whole run. A failed lookup costs the titles and nothing
else: the URL is derived locally and the exit code is already settled.

--json now emits an object per branch holding candidates, unlabelled
and duplicates arrays, in place of the bare array of verdicts.

On release-1.6 today this lists 13 unlabelled originals, the nine above
among them, and three duplicates that are open right now: the bot's
conflict drafts for #3936, #4254 and #4292 were left open next to their
hand backports, and #4254 and #4292 each also have a fork PR open next
to its reopening from a branch in this repository. #4421 is closed and
shows up only as a closed claim on #4280.

Assisted-by: LLM
Signed-off-by: Myasnikov Daniil <[email protected]>