Member Content: Courses: Improve SQL query for 6.9 compat.

[n7studios]

Summary

Testing in 6.9 RC1 raised a SQL error when using Member Content and modifying the get_adjacent_post() query to determine the previous/next page links within a course (series of WordPress pages gated by form, tag or product):

This is due to a change in the original query, before modification:

• 6.8 and lower: WHERE p.post_date < '2025-11-13 14:07:58' AND p.post_type = 'page' AND ( p.post_status = 'publish' OR p.post_status = 'private' )
• 6.9 and newer: WHERE (p.post_date < '2025-11-13 14:04:15' OR (p.post_date = '2025-11-13 14:04:15' AND p.ID < 3)) AND p.post_type = 'page' AND ( p.post_status = 'publish' OR p.post_status = 'private' )

The replacement WHERE clause was too generic in replacing part of the query on the first AND.

This PR resolves by using a more specific search and replace, and submitted here to backport test against 6.8.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

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