Validate URL components per RFC 3986: reject unencoded whitespace

[Copilot]

The simple_url parser was accepting unencoded whitespace in URL components, violating RFC 3986 which requires whitespace to be percent-encoded.

Changes

• Validation: Added whitespace checks for username, password, and path components using char::is_whitespace
• Error handling: Returns InvalidDomainCharacter for consistency with existing host validation
• Tests: Added coverage for space, tab, and newline characters in each component

Example

// Now correctly rejected
gix_url::parse("http://user name@example.com/path".into())  // Error
gix_url::parse("http://example.com/ path".into())           // Error

// Percent-encoded whitespace still accepted (correct per RFC 3986)
gix_url::parse("http://user%20name@example.com/path".into()) // Ok

The validation is applied during URL parsing in ParsedUrl::parse(), catching invalid characters before the URL components are stored.

Original prompt

Validate invariants according to the URL specification when gix_url::simple_url parses URLs. For instance, right now it allows whitespace in http:// URLs which isn't correct.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Byron]

Fine, but I realise that percent-encoding isn't handled consistently. Everything needs to be percent-decoded. Will be done in a follow-up.