chore: Fix return type for Pagination_Helper::get_page_number_from_query_loop()

[lonesahilnazir]

Context

• The Pagination_Helper::get_page_number_from_query_loop() method was documented as returning a string, but it returns an int when successfully matching a query loop page, and an empty string otherwise. This type-annotation mismatch causes static analysis violations (like Psalm/PHPStan) when consuming methods that properly declare an int return type pass this value through. This PR fixes the PHPDoc and adds unit tests to strictly pin the expected int|string return types.

Summary

This PR can be summarized in the following changelog entry:

• changelog: non-user-facing
• Corrects the return type documentation for Pagination_Helper::get_page_number_from_query_loop() to reflect its actual int|string behavior.

Relevant technical choices:

• Updated the PHPDoc to @return int|string to match what the code actually outputs.
• Added test_query_loop_page_number_returns_int_when_above_one and test_query_loop_page_number_returns_empty_string_when_absent to pin the runtime behavior, ensuring future clean-ups do not silently change the contract.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Since this is a code-level change only, verify that the automated CI pipelines (PHPUnit and PHPCS checks) pass successfully for this PR.
• Alternatively, run the test suite locally using composer test-wp-env or by running vendor/bin/phpunit tests/Unit/Helpers/Pagination_Helper_Test.php.

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.

QA can test this PR by following these steps:

• Not applicable.

Impact check

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

• No user-facing behavior impact; this purely corrects internal type-hinting for static analysis tooling.

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.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on 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 run grunt build:images and committed the results, if my PR introduces or edits 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 #