GET fallback for link checker

[fbacall]

Summary of changes

• Try a GET request if a link returns a 400 or 405 code when trying a HEAD
• Move some link checker logic out into a class

Motivation and context

Certain services will not respond to HEAD requests

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.

[Copilot]

Pull Request Overview

This PR enhances the link checker to handle services that don't respond to HEAD requests by implementing a GET request fallback. The implementation has been refactored from procedural code in a rake task into a dedicated LinkChecker class.

Key Changes:

• Added GET request fallback when HEAD requests return 400 or 405 status codes
• Refactored link checking logic into a new LinkChecker class with caching support
• Added comprehensive test coverage for the new link checker functionality

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
lib/link_checker.rb	New class implementing link checking with HEAD/GET fallback and URL caching
lib/tasks/check_urls.rake	Simplified to delegate to LinkChecker class instead of containing inline logic
test/unit/link_checker_test.rb	New test suite covering various link checking scenarios including GET fallback and redirect handling

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[Copilot]

[nitpick] The comment mentions trying GET for '400 or generic 400 error' which is redundant. Consider revising to clarify why 400 is included alongside 405. A clearer comment would be: 'Try a GET if HEAD returns Method Not Allowed (405) or Bad Request (400), as some servers reject HEAD requests'.

Suggested change

	code = get_code(url) if code == 400 || code == 405 # Try a GET if HEAD not allowed (or generic 400 error)
	code = get_code(url) if code == 400 || code == 405 # Try a GET if HEAD returns Method Not Allowed (405) or Bad Request (400), as some servers reject HEAD requests