### Godot version

v4.1.stable.official [970459615]

### System information

Windows 10

### Issue description

When changing the `file_mode` of `FileDialog` from "Open File" to "Save File" via code the "Save" button keeps disabled until a file or folder is selected if the "Open" button was disabled before.

Basically the button activity has to be updated when `file_mode` is changed. I have made the change already but the last time I opened an PR without an issue some people requested I open one. So here you are!

### Steps to reproduce

- Open the MRP and start it
- click anywere to open the dialog
- after 10 seconds the dialog changes file_mode
- if the button is disabled before the change it is as well after it

### Minimal reproduction project

[file_dialogue_test.zip](https://github.com/godotengine/godot/files/11991311/file_dialogue_test.zip)

TreeItem *ti = tree->get_selected();
	if (!ti) {
		return;
	}
	Dictionary d = ti->get_metadata(0);
	if (!d["dir"]) {
		file->set_text(d["name"]);
	} else if (mode == FILE_MODE_OPEN_DIR) {
		set_ok_button_text(RTR("Select This Folder"));
	}
	get_ok_button()->set_disabled(_is_open_should_be_disabled());
}
void FileDialog::_tree_item_activated() {
	TreeItem *ti = tree->get_selected();
	if (!ti) {
		return;
	}
	Dictionary d = ti->get_metadata(0);
	if (d["dir"]) {
		_change_dir(d["name"]);
		if (mode == FILE_MODE_OPEN_FILE || mode == FILE_MODE_OPEN_FILES || mode == FILE_MODE_OPEN_DIR || mode == FILE_MODE_OPEN_ANY) {
			file->set_text("");
		}
		_push_history();
	} else {
		_action_pressed();
	}
}
void FileDialog::update_file_name() {