List Table: Fix incorrect use of row headers in post list tables

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/32892

Changes the semantic structure so that:

• Checkbox columns use <td> instead of <th scope="row">
• Primary content columns (like post titles) use <th scope="row"> as proper row headers

This ensures screen readers announce meaningful content (post titles, usernames, etc.) as row headers instead of "Select [item]" from checkboxes.

• Centralized aria-label Handling: Added a get_primary_column_aria_label() hook directly in WP_List_Table so the parent class is responsible for attaching aria-label to primary column row headers rather than delegating to subclasses.
• Removed "Title and Title" Duplication: Overrode the method in WP_Posts_List_Table, WP_Plugins_List_Table, WP_Users_List_Table, and WP_MS_Themes_List_Table to return each item's clean, human-readable identifier. This resolves the screen-reader duplication caused by the row header's accessible name being computed from full cell content (including row actions and excerpt).
• Cleanup: Removed the unused abbr attribute as flagged by reviewers.
• CSS Consistency: Updated CSS across common.css, list-tables.css, and forms.css to add td.check-column selectors alongside the existing th.check-column rules, and extended responsive column-primary sibling selectors to cover the <th> case.

[himanshupathak95]

Other list tables, such as the Users list and multi-site tables, are yet to be updated. Additionally, the styling has been affected, which I will address in the subsequent commits.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[himanshupathak95]

The styles of the plugins list table and highly coupled, and removing padding on unselected rows is affecting other headers. Debugging deeper into this -

Screen.Recording.2025-09-09.at.15.23.37.mov

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props abcd95.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.