chore: keep internal OpenAPI

The OpenAPI definitions for endpoints that haven't been corrected yet
are still useful, though maybe not ready for public visibility. An
internal OpenAPI document is generated to preserve these definitions.

buf.gen.yaml

@@ -37,6 +37,15 @@ plugins:
       - allow_merge=true,merge_file_name=openapiv2
       - version=true
       - omit_enum_default_value=true
+  - plugin: buf.build/grpc-ecosystem/openapiv2:v2.15.2
+    out: openapiv2/internal
+    opt:
+      - output_format=yaml
+      - json_names_for_fields=false
+      - allow_merge=true,merge_file_name=openapiv2
+      - version=true
+      - omit_enum_default_value=true
+      - visibility_restriction_selectors=INTERNAL
   - plugin: buf.build/community/pseudomuto-doc:v1.5.1
     out: gen/grpc-doc
     opt: