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/host.rs

@@ -552,18 +552,6 @@ pub trait HostApi {
         let _ = tab_id;
         None
     }
-
-    /// DocApi v2 (`ocs_doc_api`): dispatch one bincode-serialized `DocApiEnvelope`
-    /// (one write op OR a read-only query batch) and return the bincode-serialized
-    /// `Result<Receipt, ApiError>`. Opaque bytes:
-    /// this crate does not depend on `ocs_doc_api`, it only routes the envelope.
-    ///
-    /// Added for the DocApi v2 adapter. The default returns `Err` ("not supported");
-    /// the in-process host (`HostSession`) overrides it to call the DocApi executor.
-    fn doc_api_dispatch(&mut self, tab_id: u64, bytes: &[u8]) -> Result<Vec<u8>, String> {
-        let _ = (tab_id, bytes);
-        Err("DocApi v2 not supported by this host".to_string())
-    }
 }
 
 /// Simplified, read-only entity kind exposed by [`DocumentReader`].