Import `WP_Error` class instead of using fully qualified references

[galatanovidiu]

Summary

Update PHPCS configuration to allow importing WP_Error via use statements instead of requiring fully qualified references (\WP_Error) throughout the codebase.

Background

During PR #123 review, @JasonTheAdams suggested importing WP_Error to drop the root namespace prefix. Investigation revealed that the current SlevomatCodingStandard.Namespaces.FullyQualifiedExceptions rule enforces fully qualified references for classes ending with Error, which includes WP_Error.

However, WP_Error is not an actual exception class (it doesn't extend Exception or Throwable) - it's a WordPress utility class for error handling. The rule's documentation explicitly supports adding such classes to ignoredNames.

Proposed Changes

1. Update phpcs.xml to add WP_Error to ignoredNames:

<rule ref="SlevomatCodingStandard.Namespaces.FullyQualifiedExceptions">
    <properties>
        <property name="ignoredNames" type="array">
            <element value="WP_Error" />
        </property>
    </properties>
</rule>

2. Update all namespaced files that use \WP_Error:
   • Add use WP_Error; import statement
   • Replace \WP_Error with WP_Error in code
   • Update PHPDoc @return types from \WP_Error to WP_Error

References

• PR [5/6] Update core layer and built-in abilities for MCP schema #123 review comment: [5/6] Update core layer and built-in abilities for MCP schema #123
• Slevomat Coding Standard docs: https://github.com/slevomat/coding-standard/blob/master/doc/namespaces.md