core(seo): improve validation for invalid canonical URLs

[emazack]

Closes #12008

Summary

This PR fixes a bug in the canonical audit where some syntactically invalid URLs (e.g., those with a space in the host like https:// example.com) were incorrectly being classified as relative URLs.

The previous validation logic was insufficient because it didn't correctly handle cases where the browser's link parser would return a non-null, but still malformed, href string.

This change replaces the old logic with a more robust, two-step validation process inside a nested try...catch block.

1. It first checks if the hrefRaw is syntactically valid at all by attempting to construct a URL with a base. If this fails, the URL is correctly marked as invalid.
2. If the first check passes, it then attempts to construct a URL without a base. If this second check fails, the URL is correctly marked as relative.
3. If both checks pass, the URL is correctly identified as a valid, absolute URL.

To support this fix, the existing unit tests have been improved:

• The test for invalid URLs, which was passing for the wrong reasons, has been updated to correctly expose the bug.
• A new test case has been added to explicitly verify the href: null scenario, improving the overall robustness of the test suite.

All related unit tests now pass with these changes.

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