Fix: A rare bug where the link contains a linebreak or whitespace causes strange url

[Grotax]

Fixes: nextcloud/news#3300

[Copilot]

Pull Request Overview

This PR fixes a bug where RSS/Atom feed links containing linebreaks or whitespace cause malformed URLs. The fix adds trim() calls to strip whitespace from link values before processing them.

• Strip whitespace from RSS link element values using trim()
• Strip whitespace from Atom link href attributes using trim()
• Add comprehensive test coverage for both whitespace handling and relative URL processing

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/FeedIo/Rule/Link.php	Trims whitespace from RSS link element values
src/FeedIo/Rule/Atom/Link.php	Trims whitespace from Atom link href attributes
tests/FeedIo/Rule/LinkTest.php	Adds test cases for whitespace handling and relative URL processing

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