resolve race condition of get_buffer_content trying to read file content before handle_open_file finished

[nohzafk]

Testing lsp-bridge in container gives me more edge case

This error happens constantly, get_buffer_content is trying to read file content before handle_open_file finished, when it happens, lsp stops working, that's why I'm having trouble using language server inside container.

We need to wrap the file access opeartion with lock to ensure the access is atomatic.

error log

Traceback (most recent call last):
  File "/tmp/lsp-bridge/lsp_bridge.py", line 625, in event_dispatcher
    getattr(self, func_name)(*func_args)
  File "/tmp/lsp-bridge/lsp_bridge.py", line 886, in _do
    action.call(name, *args)
  File "/nix/store/f8ki1wxp84nj1nldjlj50y5wm5qknyn9-lsp-bridge-src-20240717-master/core/fileaction.py", line 166, in call
    getattr(self, method)(*args, **kwargs)
  File "/nix/store/f8ki1wxp84nj1nldjlj50y5wm5qknyn9-lsp-bridge-src-20240717-master/core/fileaction.py", line 191, in change_file
    buffer_content = get_buffer_content(self.filepath, buffer_name)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/f8ki1wxp84nj1nldjlj50y5wm5qknyn9-lsp-bridge-src-20240717-master/core/utils.py", line 129, in get_buffer_content
    return lsp_bridge_server.file_server.file_dict[filename]
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: '/workspaces/the_gleam_learner/test/flat_tensor_test.gleam'

With this change, the error no long happen, and language server works good, close #997