fix: when initialize the avante, dont mark the current buffer as selected if it's a directory path

[guanghechen]

This pull request updates the Sidebar:initialize function in lua/avante/sidebar.lua to improve file selection behavior. Specifically, it adds a check to ensure the provided file path exists before adding it to the file selector.

• lua/avante/sidebar.lua: Updated the Sidebar:initialize method to verify the existence of filepath using Utils.path.is_exist_filepath before calling add_selected_file.