fix(mcp): translate OpenAPI 3.0 nullable into JSON Schema

The response schemas added yesterday made every paginated list tool fail. The
plugin's fields are documented as OpenAPI format and validated with a JSON
Schema validator, and `nullable: true` is not a JSON Schema keyword — so it is
ignored, `type: string` is enforced, and `previous: null` on the first page is
rejected as *null is not of type "string"*.

The failure lands after the backend has already done the work, surfaces to the
caller as a bare tool retry exhaustion, and appears nowhere in the gateway's own
logs. Found only because an orchestrating agent asked a question that paged.

The generator now rewrites `nullable: true` into `type: [T, "null"]` wherever it
inlines a schema, request bodies included. A 3.1 spec needs none of this.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kong/mcp/ai-mcp-proxy.yaml

@@ -58,13 +58,15 @@ config:
                   type: integer
                   example: 123
                 next:
-                  type: string
-                  nullable: true
+                  type:
+                  - string
+                  - 'null'
                   format: uri
                   example: http://api.example.org/accounts/?page=4
                 previous:
-                  type: string
-                  nullable: true
+                  type:
+                  - string
+                  - 'null'
                   format: uri
                   example: http://api.example.org/accounts/?page=2
                 results: