Allow multiple image sizes for regeneration via repeated --image_size flags

[Copilot]

• Switch --image_size to repeating flag syntax ([--image_size=<image_size>...])
• Update parsing to handle both single string (WP-CLI 2.x) and array (WP-CLI 3.x)
• Update docblock examples and Behat tests to use --image_size=large --image_size=medium
• Fix test: "thumbnail" → "thumbnails" for the no-regeneration message with multiple sizes
• Fix log message: update_attachment_metadata_for_image_size() returns string[] of actually-regenerated sizes; "Regenerated" message uses that list instead of all requested sizes
• Extract get_image_sizes_description() private helper and use Utils\pluralize() internally to eliminate both duplicated string-building logic and the manual plural noun parameter

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow multiple image_sizes for regeneration</issue_title>
<issue_description>## Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

I had the case that I needed to regenerate 2 image sizes, or rather 1 newly added and 1 existing image size.
To solve this I had to run wp media regenerate --image_size=new_image_size and wp media regenerate --image_size=updated_image_size

Since I am talking about a multisite with 6 sites that we want to update, and each has at least 10k attachments, running the two commands does not seem to be efficient.

Describe the solution you'd like

I would love to be able to run either:

Enhancing the existing --image_size to allow single or multiple image sizes)
wp media regenerate --image_size=new_image_size,updated_image_size

or

Introducing a new --image_sizes that will accept multiple, in order to keep the grammar correct.
wp media regenerate --image_sizes=new_image_size,updated_image_size

Note: I'm happy to propose a PR, I'd just need a heads up if you think it's a good addition and which of the two suggested routes to take. I might need help updating the related unit tests.</issue_description>

<agent_instructions>This is now possible, see wp-cli/wp-cli#6198</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@swissspidy Related:

• wp theme list show multiple status extension-command#224
• Allow multiple values for --field options in wp plugin list extension-command#159</comment_new>

• Fixes Allow multiple image_sizes for regeneration #216

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[codecov]

Codecov Report

❌ Patch coverage is 94.52055% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/Media_Command.php	94.52%	4 Missing ⚠️

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