Fix handling of { port: '0' }

[sambostock]

Being passed { port: '0' } ({ port: string }) is a Typescript type error, but we explicitly have a test that checks that it is gracefully handled, with behavior matching being passed

• { port: 0 } ({ port: number }),
• '0' (string), or
• 0 (number),

which all convert the port to 0.

However, that test was actually dependent on the following leak from a test above it:

process.env.HOST = 'localhost';

Switching to vi.stubEnv and ensuring to call vi.unstubAllEnvs reveals that the behavior is broken.

---

This updates the behavior to apply the same check to _userOptions.port as we do to _userOptions, and coerce number and string values to integers.