merlint sweep: ocaml-atproto-handle, ocaml-atproto-oauth
- snake_case alcotest suite names (E617).
- rename test_metadata / test_handles in interop drivers to drop the
Test_ prefix shadowed by the file's module path (E330).
- extract helpers in indigo interop test_handles to flatten 5-deep
nesting (E010).
- replace open-ended (try ... with _ -> ()) in the loopback callback
with explicit Eio.Io | End_of_file matches (E105).
- shorten validate_server_no_auth_code_grant -> ..._no_authz_grant
(E320).
- lift the Json/IO error wrappers into err_decode / err_io helpers
in Atproto_oauth.Token, replacing the inline Error (`Msg (Fmt.str ...))
pattern (E340).