Fixed a rare crash provoked by two factors:

* Fixed LSPDocumentClient::~LSPDocumentClient: It now explicitly calls `mDoc->unregisterClient(this);`. This ensures that whenever the client is destroyed—whether by the server shutting down or just the document closing—it cleanly detaches from the TextDocument.
* Fixed `TextDocument::notifyDocumentClosed` Iterator Invalidation: Since `onDocumentClosed` is called during a loop over `mClients`, and `LSPDocumentClient` now unregisters itself during that exact same loop, it was modifying the `mClients` `std::unordered_set` while it was being iterated (which is undefined behavior). I changed `notifyDocumentClosed` to iterate over a copy of the clients, safely allowing clients to unregister themselves when they receive the notification.

src/tools/ecode/plugins/lsp/lspdocumentclient.cpp

@@ -41,6 +41,7 @@ void LSPDocumentClient::setupFoldRangeService() {
 
 LSPDocumentClient::~LSPDocumentClient() {
 	mDoc->getFoldRangeService().setProvider( nullptr );
+	mDoc->unregisterClient( this );
 	mDoc = nullptr;
 	UISceneNode* sceneNode = getUISceneNode();
 	if ( nullptr != sceneNode && 0 != mTag )