Fix transform_cypher_param to pass agtype_access_operator a VARIADIC array

[vividy163]

Root cause

agtype_access_operator is declared VARIADIC agtype[]. Every other call site in cypher_expr.c packs its arguments into a single agtype[] ArrayExpr via make_agtype_array_expr() and sets funcvariadic = true on the resulting FuncExpr (see transform_cypher_map_projection, transform_cypher_indirection).

transform_cypher_param() — the path that turns a Cypher $param reference into an AGE function call — was the only exception. It passed the bound Param and the key-name Const directly as separate FuncExpr arguments without wrapping them and without setting funcvariadic.

With the Simple Query protocol PostgreSQL implicitly assembles the variadic array from separate arguments at execution time, which masked the bug. Under the Extended Query protocol (Parse/Bind/Execute prepared statements) the executor does not perform that implicit assembly, so agtype_access_operator read past its single agtype argument and treated adjacent memory as additional array elements. The resulting out-of-bounds read surfaced as corrupted relation names in "relation does not exist" error responses (random bytes mixed with fragments of in-flight query text / property values such as "November", "customer", "communication", and control bytes like 0x01). Drivers that UTF-8 decode the server error then raised:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x9e in position 51: invalid start byte

The bug reproduces with any Open/asyncpg/JDBC driver that uses prepared statements with bound agtype parameters against a Cypher query that references $param (for example MERGE (n:base {entity_id: $entity_id}) or MATCH (s:base {id: $src_id}), (t:base {id: $tgt_id}) CREATE (s)-[r:DIRECTED {…}]->(t), the shape used by LightRAG). The error content is unrelated to the payload — edges with pure-ASCII property values failed identically.

Fix

Pack the two arguments in make_agtype_array_expr() and set func_expr->funcvariadic = true, exactly matching every other call site.

newa = make_agtype_array_expr(args);
func_expr = makeFuncExpr(func_access_oid, AGTYPEOID, list_make1(newa),
                         InvalidOid, InvalidOid, COERCE_EXPLICIT_CALL);
func_expr->funcvariadic = true;

Regression test

Added p_create_edge to regress/sql/cypher_create.sql, which PREPAREs a MATCH … {id: $src_id}, … {id: $tgt_id} CREATE ()-[:e {id: $edge_id}]->() statement (the exact shape used by LightRAG/asyncpg) and EXECUTEs it repeatedly.