Add agtype <-> jsonb bidirectional casts (issue #350)

Register explicit casts between agtype and jsonb, enabling:

  SELECT properties(n)::jsonb FROM cypher(...) AS (n agtype);
  SELECT '{"key": "value"}'::jsonb::agtype;

  -- Use jsonb operators on graph data:
  SELECT (props::jsonb)->>'name' FROM cypher(...) AS (props agtype);

Implementation uses SQL language functions that go through proven
text-intermediate paths:
  agtype -> jsonb:  agtype_to_json() -> json::jsonb
  jsonb -> agtype:  jsonb::text -> text::agtype

This approach is safe because agtype extends jsonb's binary format
with types (AGTV_INTEGER, AGTV_FLOAT, AGTV_VERTEX, AGTV_EDGE,
AGTV_PATH) that jsonb does not recognize, making direct binary
conversion unreliable.  The text roundtrip handles all value types
correctly including graph types (vertex/edge properties are
extracted as JSON objects).

All 31 regression tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gregfelice

sql/agtype_coercions.sql

@@ -174,6 +174,33 @@ AS 'MODULE_PATHNAME';
 CREATE CAST (agtype AS json)
     WITH FUNCTION ag_catalog.agtype_to_json(agtype);
 
+-- agtype -> jsonb (explicit)
+-- Uses text intermediate because agtype extends jsonb's binary format
+-- with types (AGTV_INTEGER, AGTV_FLOAT, AGTV_VERTEX, AGTV_EDGE, AGTV_PATH)
+-- that jsonb does not recognize.
+CREATE FUNCTION ag_catalog.agtype_to_jsonb(agtype)
+    RETURNS jsonb
+    LANGUAGE sql
+    IMMUTABLE
+RETURNS NULL ON NULL INPUT
+PARALLEL SAFE
+AS 'SELECT ag_catalog.agtype_to_json($1)::jsonb';
+
+CREATE CAST (agtype AS jsonb)
+    WITH FUNCTION ag_catalog.agtype_to_jsonb(agtype);
+
+-- jsonb -> agtype (explicit)
+CREATE FUNCTION ag_catalog.jsonb_to_agtype(jsonb)
+    RETURNS agtype
+    LANGUAGE sql
+    IMMUTABLE
+RETURNS NULL ON NULL INPUT
+PARALLEL SAFE
+AS 'SELECT $1::text::agtype';
+
+CREATE CAST (jsonb AS agtype)
+    WITH FUNCTION ag_catalog.jsonb_to_agtype(jsonb);
+
 CREATE FUNCTION ag_catalog.agtype_array_to_agtype(agtype[])
     RETURNS agtype
     LANGUAGE c