Fix URL validation and handling in URLUtil

[sfahaddev]

Closes #12186

Describe the changes you have made here: what, where, why, ...

• What: Fixed URL validation and handling in the URLUtil class.
• Where: Updated the isURL and create methods in URLUtil and added/modified tests in URLUtilTest.
• Why: To resolve the issue where relative URLs (e.g., www.example.com) were not being handled correctly, and to ensure proper validation of URLs with various protocols (e.g., http://, https://, ftp://, file://).

Changes:

1. isURL Method:

   • Removed unnecessary exception handling (MalformedURLException and IllegalArgumentException).
   • Now only catches URISyntaxException to correctly identify invalid URLs.
   • Ensures that only absolute URLs are considered valid.

2. create Method:

   • Added logic to prepend http:// only to URLs without a protocol (e.g., www.example.com → http://www.example.com).
   • Ensures that URLs with existing protocols (e.g., ftp://example.com, file:///path/to/file) are not modified.

3. Tests:

   • Added new test cases for relative URLs, absolute URLs, and other protocols.
   • Fixed normalization issue for file URLs in tests (e.g., file:///path/to/file → file:/path/to/file).

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨