Add shortest_path / all_shortest_paths SRFs

Add two C set-returning functions that compute unweighted (hop-count)
shortest paths over the cached global graph adjacency via BFS, callable
both at the SQL top level and inside a cypher() RETURN:

  - age_shortest_path(...)       -> the single shortest path (0 or 1 rows)
  - age_all_shortest_paths(...)  -> every shortest path, one per row

The signature follows the natural Cypher argument order
(graph, start, end, edge_types, direction, min_hops, max_hops), registered
in sql/agtype_typecast.sql (install) and age--1.7.0--y.y.y.sql (upgrade).
Unimplemented parameters fail loudly: multiple relationship types and a
non-zero min_hops raise ERRCODE_FEATURE_NOT_SUPPORTED. A single edge type
(string or one-element array) is honored, and a NULL endpoint yields no
rows per Cypher null semantics (wrong-typed endpoints / NULL graph still
error).

To call the SRFs inside a cypher() RETURN, transform_cypher_return now sets
query->hasTargetSRFs (it was the only results-producing clause that didn't,
so the planner never added a ProjectSet node), and transform_FuncCall
auto-prepends the graph name for snake_case shortest_path /
all_shortest_paths. camelCase names are reserved for the future native
grammar.

Robustness:
  - BFS guards against non-existent endpoints (returns 0 rows instead of
    crashing) and honors CHECK_FOR_INTERRUPTS.
  - An unknown edge label now matches no edges instead of silently
    traversing all of them (get_label_relation returns InvalidOid).

Adds the age_shortest_path regression test (directed/undirected, label
filtering, parallel edges, self-loops, max_hops, the not-supported stubs,
NULL and non-existent endpoint/graph guards).

38/38 installcheck pass.

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

modified:   Makefile
modified:   age--1.7.0--y.y.y.sql
modified:   sql/agtype_typecast.sql
modified:   src/backend/parser/cypher_clause.c
modified:   src/backend/parser/cypher_expr.c
modified:   src/backend/utils/adt/age_vle.c
new file:   regress/expected/age_shortest_path.out
new file:   regress/sql/age_shortest_path.sql

Author: jrgemignani

Makefile

@@ -201,6 +201,7 @@ REGRESS = scan \
           cypher_delete \
           cypher_with \
           cypher_vle \
+          age_shortest_path \
           cypher_union \
           cypher_call \
           cypher_merge \

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

@@ -537,6 +537,37 @@ CALLED ON NULL INPUT
 PARALLEL UNSAFE
 AS 'MODULE_PATHNAME';
 
+-- Unweighted (hop-count) shortest path between two vertices, computed over the
+-- cached global graph adjacency via BFS. Returns a single path (0 or 1 rows).
+-- Argument order mirrors the Cypher shortestPath() pattern
+-- (a)-[:type*min_hops..max_hops]->(b):
+--   (graph_name, start, end, edge_types, direction, min_hops, max_hops)
+CREATE FUNCTION ag_catalog.age_shortest_path(IN agtype, IN agtype, IN agtype,
+                                             IN agtype DEFAULT NULL,
+                                             IN agtype DEFAULT NULL,
+                                             IN agtype DEFAULT NULL,
+                                             IN agtype DEFAULT NULL)
+    RETURNS SETOF agtype
+LANGUAGE C
+STABLE
+CALLED ON NULL INPUT
+PARALLEL UNSAFE
+AS 'MODULE_PATHNAME';
+
+-- All unweighted shortest paths between two vertices (one path per row).
+-- Same argument order as age_shortest_path.
+CREATE FUNCTION ag_catalog.age_all_shortest_paths(IN agtype, IN agtype, IN agtype,
+                                                  IN agtype DEFAULT NULL,
+                                                  IN agtype DEFAULT NULL,
+                                                  IN agtype DEFAULT NULL,
+                                                  IN agtype DEFAULT NULL)
+    RETURNS SETOF agtype
+LANGUAGE C
+STABLE
+CALLED ON NULL INPUT
+PARALLEL UNSAFE
+AS 'MODULE_PATHNAME';
+
 --
 -- Composite types for vertex and edge
 --