Fix gix credential fill to accept protocol+host without URL field

[Copilot]

Fixes #2190

Problem

gix credential fill incorrectly required the url field to be present, even when both protocol and host fields were provided. This differs from Git's behavior and breaks compatibility with credential helpers that only provide individual fields.

According to the git-credential documentation:

When this special attribute is read by git credential, the value is parsed as a URL and treated as if its constituent parts were read (e.g., url=https://example.com would behave as if protocol=https and host=example.com had been provided).

This implies that url should be optional when the constituent parts (protocol and host) are available.

Before

$ echo -e "protocol=https\nhost=github.com" | gix credential fill
Error: The url wasn't provided in input - the git credentials protocol mandates this

After

$ echo -e "protocol=https\nhost=github.com" | gix credential fill
Username for https://github.com: 

Changes

• Modified validation logic in gix-credentials/src/program/main.rs to accept either url OR both protocol and host
• Enhanced destructure_url_in_place method to construct URLs from individual components when url field is missing
• Updated gitoxide-core credential handling to work with constructed URLs
• Improved error messages to accurately describe the new validation requirements
• Added comprehensive tests covering all scenarios including edge cases

Compatibility

• ✅ Backward compatible: Existing url-based workflows continue to work unchanged
• ✅ Git compatible: Now matches git credential fill behavior exactly
• ✅ Extended support: Works with any combination of protocol, host, path, and username

All existing tests continue to pass, and 7 new test cases ensure the new functionality works correctly.

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