Add shortest_path / all_shortest_paths SRFs

[jrgemignani]

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).
• manage_GRAPH_global_contexts no longer self-deadlocks: its process-local mutex is released via PG_TRY/PG_CATCH if the critical section errors.

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). 37/37 installcheck pass.

Co-authored-by: Claude noreply@anthropic.com

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

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