Fix parse-tree malformation in OPTIONAL MATCH WHERE re-attach (#2378)

transform_cypher_optional_match_clause was calling transform_cypher_expr
with EXPR_KIND_JOIN_ON when re-attaching the detached WHERE as the
LEFT JOIN's ON condition.  All other WHERE transforms in cypher_clause.c
use EXPR_KIND_WHERE, and there are three explicit
p_expr_kind == EXPR_KIND_WHERE guards (cypher_clause.c:5415, 5679, 6597)
that do load-bearing variable resolution for sub-pattern predicates --
walking up parent parsestates to rebind variables like `friend` inside
EXISTS { (friend)-[...]->(...) }.

Using EXPR_KIND_JOIN_ON bypassed those guards, so the sub-pattern fell
through to the "create new variable" path and produced a structurally
invalid parse tree.  Under a release PG build the query happened to
produce correct-looking output, but under --enable-cassert the
downstream invariant checks aborted, crashing the backend and taking
down the regression run (reported by @MuhammadTahaNaveed).

Fix: use EXPR_KIND_WHERE, matching the pattern already established in
transform_cypher_clause_with_where at line 2619.

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

Author: gregfelice

src/backend/parser/cypher_clause.c

@@ -2872,7 +2872,7 @@ static RangeTblEntry *transform_cypher_optional_match_clause(cypher_parsestate *
         Node *where_qual;
 
         where_qual = transform_cypher_expr(cpstate, saved_where,
-                                           EXPR_KIND_JOIN_ON);
+                                           EXPR_KIND_WHERE);
         where_qual = coerce_to_boolean(pstate, where_qual, "WHERE");
         j->quals = where_qual;
     }