Migration/AddLimitToTextColumns: Improve code coverage and refactor cop

What does this MR do and why?

This MR is a follow-up of !143374 (merged) and refactors Migration/AddLimitToTextColumns to use node patterns where possible to improve readability.

It also adds missing examples:

• When limit: is not supported
• When table method is unknown

Contributes to:

• #440164
• #355012

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

bundle exec rubocop --parallel --only Migration/AddLimitToTextColumns db/ ee/db/
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