Issue:
There has been an increase in the number of compatibility issues due to data format changes in WT-15002, WT-15003, and WT-. Today, we only have compatibility testing between released versions for attached storage (ASC). For Disagg, there is no release branch yet, so we do not test forward or backward compatibility between releases or commits. We have also learned that IB clusters may move backwards and forwards between commits on mongo master. This means clusters can easily break if there are compatibility issues between commits.

There is a larger umbrella ticket, WT-15897, where we will aim to better define a longer-term strategy for compatibility testing. In the short term, we need a simple and effective way to catch compatibility regressions early, especially similar to those introduced in recent months.

Scope
For now, we will implement a basic compatibility check between commits on develop as part of PR testing.

The approach follows the first strategy mentioned by Keith in WT-15897:

• In PR testing, validate compatibility against the current head of develop.
• Test both forward and backward compatibility using test/format
• This testing will run only in PRs
• The goal is to catch obvious compatibility regressions introduced by recent changes, especially data format changes.

Out of scope
This ticket does not aim to solve long-term compatibility strategy or release-based compatibility for Disagg. That work remains tracked under WT-15897.

Definition of Done

• PR testing includes a compatibility check against the head of develop.
• Both forward and backward compatibility are validated using test format.
• The changes help catch compatibility issues similar to those seen in WT-15002, WT-15003, and WT-XXX.