Reject whitespace in URL hostnames per RFC 3986

[Copilot]

gix_url::parse() incorrectly accepted URLs with whitespace in hostnames (e.g., "http://has a space"), diverging from RFC 3986 and the standard url crate behavior.

Changes:

• Modified simple_url::normalize_hostname() to reject any whitespace characters (space, tab, newline, etc.) in hostnames
• Added test coverage for whitespace validation in both integration and unit tests

Example:

// Before: incorrectly succeeded
gix_url::parse("http://has a space".into())  
// ✓ Ok(Url { host: Some("has a space"), .. })

// After: correctly rejects
gix_url::parse("http://has a space".into())
// ✗ Err(Url { source: InvalidDomainCharacter, .. })

Original prompt

#2355 Fix this - be sure to create a failing test first.

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[Byron]

When merged, there will be a follow-up that also validates that certain characters in the hier portion aren't allowed.