Docs: Add missing @return tag to WP_Image_Editor_Imagick::remove_pdf_alpha_channel()

[csemazharul]

Trac ticket: https://core.trac.wordpress.org/ticket/65916

WP_Image_Editor_Imagick::remove_pdf_alpha_channel() returns a WP_Error if Imagick::setImageAlphaChannel() throws, but there's no @return tag in the DocBlock. Been like that since the method was added in changeset 56271 (Trac #39216).

/**
 * Removes PDF alpha after it's been read.
 *
 * @since 6.4.0
 */
protected function remove_pdf_alpha_channel() {
	...
	} catch ( Exception $e ) {
		return new WP_Error( 'pdf_alpha_process_failed', $e->getMessage() );
	}
}

Found it while looking at Trac #65817. PHPStan's return.missing rule didn't catch it because there's no declared return type, so it got skipped even though set_imagick_time_limit() in the same file is being fixed for that rule in #13082.

This just adds the tag. Nothing else changes, and PHPCS output for the file is the same before and after.

One other thing I ran into: the only caller, on line 372, throws the return value away, while pdf_load_source() and ->valid() right next to it are both checked. Not sure what the right call is there. Returning the error from load() would kill the PDF preview completely instead of just giving you one with a black background, which seems worse. Or the WP_Error could come out since nothing reads it. Left it alone for now, can do either if someone has a preference.

Drafted Commit Message

Code Quality: Document `WP_Image_Editor_Imagick::remove_pdf_alpha_channel()` return value.

The `WP_Image_Editor_Imagick::remove_pdf_alpha_channel()` method has returned a `WP_Error` object when `Imagick::setImageAlphaChannel()` throws ever since it was introduced in r56271, but its DocBlock declared no return value at all. Document it as `null|WP_Error` and add the explicit `return null;` that the annotation implies.

The union deliberately uses `null` rather than `void`. In PHP's type system `void` means a function returns no value, so it cannot be a member of a union; core removed the construct across `wp-includes` and `wp-admin` in r62178 and r62179. The spelling also matters for static analysis here: PHPStan silently normalizes `void|WP_Error` to `WP_Error|null` and skips the check, whereas `null|WP_Error` correctly reports `return.missing` for a method that otherwise falls off the end.

The sole caller, in `WP_Image_Editor_Imagick::load()`, continues to discard the return value. That is intentional: surfacing the error would abort PDF preview generation outright rather than degrade it to a preview with a black background. No behavior changes.

Developed in https://github.com/WordPress/wordpress-develop/pull/13200.
Follow-up to r56271, r62178.

Props mazharul78, westonruter.
See #65817.
Fixes #65916.

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Props mazharul78, westonruter.

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[westonruter]

I'm going to change this to null|WP_Error for the reasons in Core-64694.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 63369
GitHub commit: 2043dd6

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.