`register_server_capabilities` / `unregister_server_capabilities` had no
arm for `textDocument/documentLink`, so when a server saw our
`documentLink.dynamicRegistration` capability and chose to register the
provider dynamically, the registration silently fell into the `unhandled
capability registration` warning. `document_link_provider` stayed
`None`, `GetDocumentLinks::check_capabilities` returned false, and no
`textDocument/documentLink` request was ever sent.

Follow-up to https://github.com/zed-industries/zed/pull/56011

Release Notes:

- N/A

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>