fix: settle couchbase seed before asserting unfiltered count

[babltiga]

Problem

CouchbaseQueryEngineIntegrationTest.countAffectedRowsCountsUpdateWithoutWhereAsWholeCollection
is flaky: it asserts 3 seeded documents but intermittently reads 1 or 2. Because the couchbase
engine job runs on any backend/** change, this fails unrelated PRs — it took down
#717 (a Slack SDK bump) and the ArchUnit bump in the same CI wave, with different wrong values
(1 and 2 respectively).

Root cause

Not scan consistency — every executor path shares one options() helper and the test runs
REQUEST_PLUS throughout. The differentiator is the query shape.

CouchbaseCountRewriter turns the WHERE-less UPDATE _default SET bonus = 1 into a bare
SELECT COUNT(*) AS af_count FROM _default. Verified against a real 7.6.2 container:

statement	plan operators
SELECT COUNT(*) FROM itest	CountScan
SELECT COUNT(*) FROM itest WHERE team = "eng"	PrimaryScan3 + Fetch + Filter

The CountScan fast path can read a just-committed INSERT as short even under request_plus;
driving that loop directly reproduced a stale COUNT=1, while the filtered shape never went
stale. That is exactly why this is the only count test in the class that flakes — every other one
carries a WHERE clause.

Fix

Settle the @BeforeEach seed on the same probe before any assertion runs, bounded at 30s with a
clear failure message. Since it sits in @BeforeEach it also removes the unsettled
DELETE-then-INSERT window for all 32 tests in the class, not just this one.

Deliberately test-only: the engine already requests the strongest consistency Couchbase
offers, and touching src/main/ — even a comment, which shifts the LineNumberTable — would
change the shaded jar bytes and force a connectors/couchbase/connector.json SHA-256 re-pin.

Verification

• mvn -f engines/couchbase/pom.xml clean verify — green (32/32 IT, 2/2 failsafe)
• IT re-run 3 further times on a clean tree — 4/4 green overall
• node .github/scripts/check-engine-pins.mjs couchbase — pin unchanged, confirming the shaded
  jar is byte-identical