Add URL assertion helpers and unit tests to hammy

[nfisher]

Motivation

• Provide first-class URL assertions to the hammy assertion library so tests can validate URL components directly.
• Allow creating assertions from both *url.URL and raw string inputs with clear validation errors for invalid or nil URLs.
• Improve failure diagnostics by returning descriptive messages for scheme, host, path, query, and full-string mismatches.

Description

• Added hammy/url.go which implements URL, ParseURL, and the URLAssert type with methods Scheme, Host, Path, RawQuery, QueryParam, NoQueryParam, and String using the standard net/url package.
• Implemented validation helpers invalidURL and ready and integrated with existing AssertionMessage/Assert flow to produce consistent error messages.
• Added comprehensive unit tests in hammy/url_test.go covering success and failure cases, including nil and invalid URL handling and query parameter edge cases.

Testing

• Added unit tests in hammy/url_test.go that exercise all assertion methods and negative paths using eye.Spy for failure inspection.
• Ran the package tests with go test ./... which executed the new URL tests along with existing tests.
• All automated tests completed successfully.

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Codex Task