Emit SEMI join inner conditions in the ON clause

[JoshDreamland]

A SEMI or ANTI join uses its ON clause as the match test, so every condition that references the inner relation must appear there. ClickHouse's LEFT SEMI JOIN exposes one arbitrary matching row to the WHERE clause. Testing an inner condition against that row can miss another qualifying match and return too few rows.

Do not rely on PostgreSQL's is_pushed_down flag to place these conditions. PostgreSQL sets it for semijoin conditions because their placement does not affect its executor, but the distinction matters to
a remote ClickHouse join. Move every condition that references the inner relation into the remote ON clause.

ClickHouse versions before 26.3 reject non-equality ON conditions that span both sides when join_use_nulls is enabled; 23.x rejects them regardless of that setting. Keep affected joins local on those servers. Resolve the user mapping as the executor would, and disable pushdown if no mapping is available.

Add EXISTS and NOT EXISTS tests that verify results whether the join runs remotely or locally.

Claude Summary

An EXISTS whose correlation condition references both sides was deparsed into the WHERE clause instead of the join's ON clause, silently returning too few rows. ClickHouse's LEFT SEMI JOIN surfaces the column values of one arbitrary matching row, so the condition interrogated that row rather than asking whether any matching row satisfied it.

SELECT count(*) FROM orders o WHERE EXISTS (
  SELECT 1 FROM lineitem l WHERE l.l_orderkey = o.o_orderkey AND l.l_suppkey <> o.o_custkey);

	pushed down	local
above query (TPC-H SF1)	1499990	1499998
TPC-H Q21, EXISTS-only shape	56824	73089

No NULLs are involved — every column above is NOT NULL, verified via attnotnull. It is purely the scope of the existential quantifier, demonstrable in ClickHouse with no nullable types at all:

-- right side has two matches for k=1: w=10 and w=20
LEFT SEMI JOIN ... ON l.k = r.k              -- surfaces w = 10 (one arbitrary match)
  ... ON l.k = r.k          WHERE r.w <> 10  -- 0 rows  <- wrong
  ... ON l.k = r.k AND r.w <> 10             -- 1 row   <- correct