fix: Relationship ordering in the pushdown, and two corRest tests

[kzangeli]

⚠️ Needs corNgsild#12 and corNgsild#13 merged first — the matrix's Relationship rows and url_param_join's expects are pinned against both.

Relationship ordering (§ 7.2.3.3)

The instance object is $$kv.v and carries its own type, so excluding a Relationship is one more conjunct. Two placements, and only one is right:

• for the ordering operators, inside the per-instance predicate
• for !~=, outside the negation — a guard inside the predicate that !~= negates turns "this target is excluded" into "this target matches". That's the third time today this trap has come up.

The string guard beside it had to widen at the same time, from "the target is a string" to "the target is a string or an array with a string in it", or A!~=urn dropped ["aaa","mid","zzz"] on mongoc alone.

Legacy itemType retired from both consumers

mongocEntityQuery and troeQTreeToSql now read itemTypeV directly. The fallback they carried was already dead: parseValueList is the only place that builds an LdQValueList, and it always allocates the per-item types. corNgsild can drop the field once this merges.

Two corRest tests — and only one covers what it looks like

test	covers
distop_query_big_response	a forwarded query whose response outgrows the engine's 8 KB read buffer. Verified to reach the doubling — instrumented, fires once
distop_query_pooled_connection_reuse	a forwarded query survives the Context Source restarting, pooled socket and all

⚠️ The second does not cover the fromPool && !retried retry blocks, and its header says so. Instrumenting proved it: the second query does take the pooled socket (POOLGET hits, so the liveness probe doesn't catch the death either — the peer's FIN hasn't arrived when poll() runs), and yet neither retry block is entered and the request succeeds anyway. It passes with both retry blocks compiled out, and with the probe compiled out.

Whatever recovers it, it is not those two, and they stay uncovered. Worth knowing before someone reads that test as covering the retry — finding it out took making each change and running it.

Verification: 640/640 mongoc, 590/590 corDB.