Fix undefined property warning in Item::__call magic method

[pfefferle]

Summary

• Add property_exists() checks before accessing dynamic properties in get, has, and add methods
• Prevents PHP warnings when methods like add_category() are called for properties that don't exist on the class

Test plan

• Verify no more "Undefined property" warnings at /wp-json/webmention/1.0/endpoint
• Test webmention processing still works correctly

Fixes #541

[Copilot]

Pull request overview

This PR fixes undefined property warnings that occur in PHP when the Item::__call magic method attempts to access properties that don't exist on the class, such as category. The fix adds property_exists() checks before property access in the getter, checker, and adder methods.

• Adds property_exists() guards to prevent undefined property warnings in magic method handlers
• Uses proper short-circuit evaluation to ensure properties are only accessed when they exist
• Maintains backward compatibility by preserving the same behavior while eliminating warnings

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
includes/Entity/class-item.php	Adds property_exists() checks in get, has, and add magic method branches to prevent accessing undefined properties
readme.md	Documents the bug fix in version 5.5.1 changelog

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[dshanske]

This works.