remove ocs_doc_api crate and DocApi v2 host integration

- Drop crates/ocs_doc_api from workspace and host dependency.
- Remove src/app/doc_api.rs and its module declaration.
- Strip DocApi v2 undo/dispatch plumbing from HostSession.
- Remove HostApi::doc_api_dispatch default method.
- Remove PluginRequest::DocApiRequest / PluginResponse::DocApiResponse.
- Revert plugin API version from 6 back to 5.

crates/ocs_plugin_api/src/ipc/server.rs

@@ -77,9 +77,5 @@ pub fn handle_plugin_request(
         DocumentPath { tab_id } => PluginResponse::DocumentPath(
             host.document_path(tab_id).map(|path| path.into_os_string()),
         ),
-        DocApiRequest { tab_id, bytes } => match host.doc_api_dispatch(tab_id, &bytes) {
-            Ok(bytes) => PluginResponse::DocApiResponse { bytes },
-            Err(msg) => PluginResponse::Error(msg),
-        },
     }
 }