Add GROUP BY and ORDER BY optimization

[jrgemignani]

NOTE: This PR was created with AI tools and a human.

Add GROUP BY and ORDER BY optimization for vertex/edge field access.

Transform expressions like age_id(_agtype_build_vertex(id, label, props)) into graphid_to_agtype(id) in GROUP BY and ORDER BY clauses, avoiding unnecessary vertex/edge reconstruction when only the ID is needed.

Implementation:

• Add optimize_sortgroupby_vertex_access() in cypher_clause.c
• Walk target entries with non-zero ressortgroupref (GROUP BY/ORDER BY refs)
• Detect outer accessor functions: age_id, age_start_id, age_end_id, age_properties
• Match inner build functions: _agtype_build_vertex, _agtype_build_edge
• Extract the relevant field directly and wrap with graphid_to_agtype()
• Add resjunk target entries to subquery for direct field access

Supported patterns:

• GROUP BY id(v) -> Group Key: graphid_to_agtype(v.id)
• GROUP BY start_id(e) -> Group Key: graphid_to_agtype(e.start_id)
• GROUP BY end_id(e) -> Group Key: graphid_to_agtype(e.end_id)
• ORDER BY id(v) -> Sort Key: graphid_to_agtype(v.id)
• ORDER BY start_id(e) -> Sort Key: graphid_to_agtype(e.start_id)
• ORDER BY end_id(e) -> Sort Key: graphid_to_agtype(e.end_id)
• Combined ORDER BY + GROUP BY

This complements existing optimizations:

• cypher_expr.c: optimize_vertex_field_access() for direct FuncExpr patterns
• cypher_clause.c: optimize_qual_expr_mutator() for WHERE/join conditions

Existing regression tests were not affected.
Added additional regression tests.

modified: regress/expected/unified_vertex_table.out
modified: regress/sql/unified_vertex_table.sql
modified: src/backend/parser/cypher_clause.c