Nice to have: add excerpts to postspages if available

[thijsoo]

Context

• We want to add a description to links when they are possible.

Summary

This PR can be summarized in the following changelog entry:

• Adds descriptions for the posts listed in the llms.txt file, using their excerpts when those are explicitly set.

Relevant technical choices:

• It's only added for posts since tags and pages do not have excerpts.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• With this PR, check which posts are in your llms.txt file and edit one of the posts.
• Add a custom excerpt to that post and save.
• For another post, have no excerpt but make sure to have some content.
• For another post, have no excerpt and no content either.
• Regenerate your llms.txt with the cron job or by toggeling the feature of and on. and make sure that for all posts with a custom excerpt there is a : <text> part after its link. For posts that have no custom excerpt, there's no : <text> part, even for those that have content.
• After checking this remove the excerpt from your post and make regenerate the file again. Make sure the excerpt is gone in the file and the : is also gone.
• Which means that for posts that have an explicit excerpt set, the link items look like this:

- [Link title](https://link_url): Optional link details

• And for posts that have not an explicit excerpt set, the link items look like this:

- [Link title](https://link_url)

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:

• Add content in the llms.txt file #22257 (specifically, the post lists and wherever a link is used in other places, which should not have a : <text> suffix)

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

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.

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 ccbd1e1cab42cb6a9251607a490144f960c575cb

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 25 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 52.288%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/llms-txt/domain/markdown/items/link.php	0	9	0.0%
src/llms-txt/infrastructure/markdown-services/content-types-collector.php	0	16	0.0%

Totals
Change from base Build d51584948daa306f55e437e459c53f16647f62be:	-0.002%
Covered Lines:	29181
Relevant Lines:	56965

---

💛 - Coveralls

[github-actions]

A merge conflict has been detected for the proposed code changes in this PR. Please resolve the conflict by either rebasing the PR or merging in changes from the base branch.

[leonidasmi]

CR: 🏗️

[leonidasmi]

CR: 🏗️ A final thing to be addressed.

[leonidasmi]

CR and acceptance is ✅

(I pushed a small change about the type of the $description property in its docs.

(And another one, where I removed the now-redundant string helper from the content types collector)

(I tested after those changes)