feat(mr): support direct MR ID input in mr checkout command

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Description

• test: extracting MR IDs from various URL formats and checkout tests for merge request URLs

• feat: enhance checkout command to accept merge request URLs

The changes allow users to checkout merge requests using either a direct MR ID number or a full GitLab URL. Previously, using just the ID number would fail, requiring users to always provide the full URL.

Examples of supported formats:

glab mr checkout 123
glab mr checkout https://gitlab.com/org/repo/-/merge_requests/123

Related Issues

Resolves #7684 (closed)

How has this been tested?

• Added test cases in mr_checkout_test.go to verify:
  • Direct MR ID input (e.g., "123")
  • Full GitLab URL with merge_requests format
  • Full GitLab URL with merge-requests format (hyphenated)
  • Invalid URL formats
  • Edge cases (empty input, non-numeric IDs)
• Tested manually with both formats to ensure proper checkout functionality
• Verified existing URL-based checkout functionality remains unchanged

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap