Fix invalid static request URLs when DISABLE_NGINX=1

[yohamta0]

This fixes a crash in PoWFaucet's built-in static file server when DISABLE_NGINX=1.

A request target like // makes new URL() throw while the faucet is resolving the static file path. The exception escapes the request handler and is picked up by PoWFaucet's uncaughtException handler, which shuts the process down with exit code 1 instead of returning an HTTP response.

This change:

• returns 400 Bad Request when the static request URL cannot be parsed
• keeps normal static file requests unchanged
• adds a regression test for the // request

[yohamta0]

Hi @pk910, we ran into a crash in the built-in static server when it receives an invalid request URL like //. This PR adds a small guard for that path and a regression test. Could you take a look when you have a chance? Thank you!