feat(plugins): expose document path to plugins via HostApi

OpenCADStudio/plugins#901 (comment)

- Add HostApi::document_path(tab_id) returning Option<PathBuf>.
- Add PluginRequest::DocumentPath and PluginResponse::DocumentPath for
  out-of-process plugins.
- Implement the request in the IPC server and in both V3/V4 plugin-side
  HostApi proxies.
- Implement HostSession::document_path by looking up the tab by id and
  returning its current_path.

crates/ocs_plugin_api/src/ipc/protocol.rs

@@ -136,6 +136,8 @@ pub enum PluginRequest {
     CloseDocumentViewV4 { tab_id: u64 },
     /// V4: ask the host for the stable tab identifier of the active tab.
     GetTabId,
+    /// V4: ask the host for the filesystem path of the document in `tab_id`.
+    DocumentPath { tab_id: u64 },
 }
 
 /// Responses the host sends back for `PluginRequest`.
@@ -160,6 +162,8 @@ pub enum PluginResponse {
     },
     /// V4: stable tab identifier of the active tab.
     TabId(u64),
+    /// V4: filesystem path of the document in the requested tab, if any.
+    DocumentPath(Option<String>),
 }
 
 /// Messages sent from the host to the plugin runner.