Insufficient link validation for external URLs in link widget

adding patch working patch for drupal 10.3.

The Needs Review Queue Bot → tested this issue.

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I closed 🐛 Link field doesn't sufficiently validate input Needs work as a duplicate. I updated the issue summary with info uncovered by that issue.

In the previous patch the URL constraints failed because the value was passed directly to the parent validator.

Updates in this patch:

1. Created a Symfony Url constraint object and passed it to the parent validate().
2. Added allowed protocols and used the child constraint message.
3. Applied the constraint only for external URLs.
4. Added test coverage for invalid characters, multiple comma-separated URLs, and other edge cases.

This ensures external links are properly validated and the behavior is covered by tests.

@riyas_nr I haven't done an in-depth review of the MR yet, but I notice we're adding a condition for mailto: URLs. There's no test coverage for it though. Is it possible to add a couple of test cases to LinkItemUrlValidationTest::getTestLinks() for them?

Added test coverage for mailto scheme. Moving to NR.