Add reduce() list folding function

Implement the openCypher reduce(acc = init, var IN list | body) expression,
which folds an arbitrary expression over a list, threading an accumulator
across the elements in list order. This closes a long-standing gap (reduce()
was previously unsupported) and works both at the SQL top level and inside a
cypher() RETURN/WHERE.

Implementation
--------------
reduce() is desugared, at transform time, into a correlated scalar subquery
over a new ordered aggregate rather than a new executor node, so no PostgreSQL
core changes are required:

    CASE WHEN list IS NULL THEN NULL
         ELSE COALESCE((SELECT ag_catalog.age_reduce(
                            <init>, '<serialized-body>'::text,
                            r.elem ORDER BY r.ord)
                        FROM unnest(<list>) WITH ORDINALITY AS r(elem, ord)),
                       <init>)
    END

  - A new cypher_reduce extensible node carries the accumulator/element names
    and the init/list/body expressions (grammar production, keyword, and the
    copy/out/read serialization plumbing).
  - The fold body is transformed against a throwaway two-column agtype
    namespace, its accumulator and element Var references are rewritten to
    PARAM_EXEC params 0 and 1, and it is serialized with nodeToString() into a
    text argument.
  - age_reduce_transfn (a custom agtype aggregate transition function)
    deserializes and compiles the body once per group with ExecInitExpr, then
    evaluates it per element with ExecEvalExpr, rebinding the two params. The
    body is normalized to agtype at transform time so a boolean or other
    non-agtype result cannot be misread as a by-reference Datum.

Semantics
---------
  - List order is preserved (unnest WITH ORDINALITY + aggregate ORDER BY).
  - An empty list yields the initial value; a NULL list yields NULL.
  - The list and initial value may reference outer-query variables (e.g.
    reduce(total = 0, n IN nodes(p) | total + n.age)); the body may reference
    only the accumulator and element.
  - Arithmetic, string, list-building, boolean/comparison (AND/OR/=/>), CASE,
    and element property-access bodies are all supported.
  - Outer-variable, query-parameter, nested-reduce, and aggregate references
    inside the body raise a clean ERRCODE_FEATURE_NOT_SUPPORTED error.
  - reduce is registered as a safe keyword so it remains usable as a property
    or map key, preserving backward compatibility.

Tests
-----
Adds the age_reduce regression test (registered in the install SQL and the
upgrade template so age_upgrade passes), covering: arithmetic/product/string
folds; order sensitivity; empty/NULL list; NULL element and NULL init;
list-building and CASE bodies; boolean and comparison bodies; element property
access; multiple and nested (in list/init) reduce(); reduce() in boolean
expressions, WHERE, and list comprehensions; folds over collected nodes and
node list properties; the not-supported rejections; and reduce as a map key.
All multi-row results are ordered. 38/38 installcheck pass.

Future work
-----------
The body restriction (accumulator and element only) is a property of the
standalone expression evaluation and can be relaxed without core changes:
  - Allow loop-invariant outer-variable and cypher $parameter references in
    the body by capturing them as additional eager aggregate arguments bound
    to extra param slots.
  - Support a nested reduce() inside the body via an SPI-based evaluation
    fallback for subquery-bearing bodies.
Aggregates inside the body remain intentionally unsupported, matching the
openCypher specification.

Co-authored-by: Copilot <copilot@github.com>

modified:   Makefile
modified:   age--1.7.0--y.y.y.sql
new file:   regress/expected/age_reduce.out
new file:   regress/sql/age_reduce.sql
modified:   sql/age_aggregate.sql
modified:   src/backend/nodes/ag_nodes.c
modified:   src/backend/nodes/cypher_copyfuncs.c
modified:   src/backend/nodes/cypher_outfuncs.c
modified:   src/backend/nodes/cypher_readfuncs.c
modified:   src/backend/parser/cypher_analyze.c
modified:   src/backend/parser/cypher_clause.c
modified:   src/backend/parser/cypher_gram.y
modified:   src/backend/utils/adt/agtype.c
modified:   src/include/nodes/ag_nodes.h
modified:   src/include/nodes/cypher_copyfuncs.h
modified:   src/include/nodes/cypher_nodes.h
modified:   src/include/nodes/cypher_outfuncs.h
modified:   src/include/nodes/cypher_readfuncs.h
modified:   src/include/parser/cypher_kwlist.h

Author: jrgemignani

Makefile

@@ -179,6 +179,7 @@ REGRESS = scan \
           jsonb_operators \
           list_comprehension \
           predicate_functions \
+          age_reduce \
           map_projection \
           direct_field_access \
           security \

age--1.7.0--y.y.y.sql

@@ -800,3 +800,25 @@ ALTER OPERATOR ag_catalog.?&(agtype, text[])
     SET (RESTRICT = contsel, JOIN = contjoinsel);
 ALTER OPERATOR ag_catalog.?&(agtype, agtype)
     SET (RESTRICT = contsel, JOIN = contjoinsel);
+
+--
+-- reduce(acc = init, var IN list | body) fold support
+--
+-- Transition function for the age_reduce aggregate. The fold body is compiled
+-- by transform_cypher_reduce() with the accumulator and element rewritten to
+-- PARAM_EXEC params 0 and 1 and serialized into the text argument; the
+-- transition evaluates it for each element in list order. It must be callable
+-- with a NULL transition state (no initcond), so it is intentionally not STRICT.
+CREATE FUNCTION ag_catalog.age_reduce_transfn(agtype, agtype, text, agtype)
+    RETURNS agtype
+    LANGUAGE c
+PARALLEL UNSAFE
+AS 'MODULE_PATHNAME';
+
+-- aggregate definition for reduce(); direct arguments are
+-- (init, serialized-body, element), with the element fed ORDER BY ordinality.
+CREATE AGGREGATE ag_catalog.age_reduce(agtype, text, agtype)
+(
+    stype = agtype,
+    sfunc = ag_catalog.age_reduce_transfn
+);