Make all Url fields public and replace unreachable! with proper error handling

[Copilot]

This PR makes all Url struct fields public to allow direct manipulation of URL components, while ensuring the codebase handles invalid URL states gracefully.

Changes Made

Public Fields

• Made all Url fields (scheme, user, password, host, serialize_alternative_form, port, path) public for direct access and modification

Documentation

• Added comprehensive mutability warning explaining that URLs should be parsed from string/byte parameters rather than accepted as instances and reconstructed
• Clarified the security implications of direct field manipulation

Error Handling

• Replaced unreachable! calls in URL serialization methods with proper std::io::Error::other() for invalid user-without-host combinations
• Reverted unnecessary changes to From<Infallible> implementation (keeping original unreachable! as it's actually unreachable)
• Reverted test changes (keeping original test code unchanged)
• Updated error creation to use std::io::Error::other() where possible

Now that URL fields are public and can have any state, the codebase handles URL serialization edge cases with appropriate errors instead of panics, while maintaining the existing behavior for truly unreachable code paths.

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[Byron]

@copilot Make clear that the URL to accept in parameters should always be parsed to this type, but never be an instance of this type.

[Copilot]

Updated the mutability warning to clarify that URLs should always be parsed to this type from string or byte parameters, but never be accepted as an instance of this type and then reconstructed. Commit: f1e98e5

[Byron]

@copilot Find all occurrences of unreachable! in the gix-url crate and replace it with errors. Now that the URL can have any state, we can't panic anymore.

[Copilot]

@copilot Find all occurrences of unreachable! in the gix-url crate and replace it with errors. Now that the URL can have any state, we can't panic anymore.

I've found and replaced all 4 occurrences of unreachable! in the gix-url crate with proper error handling:

1. parse.rs: Added InvalidUrlStructure error variant and updated From<Infallible> implementation
2. lib.rs (2 cases): Replaced user-without-host unreachable cases in both canonical and alternative form serialization with std::io::Error
3. tests/expand_path.rs: Updated test to handle unexpected user states gracefully

All tests pass and the workspace builds successfully. Commit: fbe454e

[Byron]

@copilot use std::io::Error::other() where possible.

[Copilot]

Updated both error creation sites to use std::io::Error::other() instead of std::io::Error::new(). Commit: c29bd0a

[Byron]

@copilot Remove this change - this unreachable! is actually unreachable.

[Copilot]

Reverted the changes to parse.rs - the unreachable! in From<Infallible> implementation is kept as originally intended since it's actually unreachable. Commit: c29bd0a

[Byron]

@copilot remove change - don't touch tests.

[Copilot]

Reverted the test changes in expand_path.rs - the original test with unreachable!("we have a name") is restored. Commit: c29bd0a