Add an Admin v2 ConnectRPC Endpoint

This skill walks through every layer touched by a new admin v2 RPC: the .proto, the Bazel build, the C++ service implementation, registration on the admin server, and the Python bindings consumed by ducktape tests.

For generic protobuf authoring conventions (field options, message design, well-known types), the canonical reference is proto/redpanda/README.md. This skill assumes you've read that and focuses on what's specific to admin v2.

Reference example

Commit 04d3160c3a (cluster: add manual upgrade finalization with admin v2 RPC) is the cleanest minimal example — a new service with a single RPC. Skim these when unsure:

• proto/redpanda/core/admin/v2/features.proto — the proto
• proto/redpanda/core/admin/v2/BUILD — the BUILD entries (look at the features_proto / features_redpanda_proto pair)
• src/v/redpanda/admin/services/features.h and features.cc — the C++ impl
• src/v/redpanda/admin/services/BUILD — the features cc_library target
• src/v/redpanda/application_admin.cc — the add_service registration

git show 04d3160c3a will print the full diff.

Procedure