Create llms.txt file when completing the task

[leonidasmi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Fixes an unreleased bug where the llms.txt file would not be created when the user completed the respective task from the task list.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Make sure you have the llms.txt feature disabled and then go to the task list
• Find the llms.txt task and click on its Enable llms.txt file
  Without this PR:
• The setting would be enabled but the file wouldn't be created nor the wpseo_llms_txt_population scheduled action would be created.
  With this PR:
• The setting would be enabled and the file would be created and the wpseo_llms_txt_population scheduled action would be created.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have ran grunt build:images and commited the results, if my PR introduces new images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #

[coveralls]

Pull Request Test Coverage Report for Build a3d480fd09efd8fc854520a0948a9c3994e98321

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 52.655%

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Totals
Change from base Build f25d8d99516e47a441deba0dccbae3df95436b6e:	0.0%
Covered Lines:	31812
Relevant Lines:	60621

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💛 - Coveralls

[thijsoo]

CR + ACC 👍