lsp diagnostics does not work for scripts without an extension

### Describe the Bug

Pyrefly language server does not provide any diagnostics for python scripts using shebang instead of .py extension.

For comparison, some other language servers, such as pyright and ty, were able to provide diagnostics for such files.

### Example
In vscode when `python.pyrefly.typeCheckingMode` is set to `default`, the language server will provide diagnostics for a python file named `script.py` but not for a python file named `script`.

### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_