describe current behavior of adding tests to versioned models

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

open to ideas! maybe here -> https://docs.getdbt.com/docs/mesh/govern/model-versions?version=1.12

### What part(s) of the page would you like to see updated?

Can we add some documentation for how data tests work with versioned models?

Current behavior: if you set a default data_tests at the top-level, it’s applied to all model versions, _unless you then_ override data_tests for a specific model version

### Additional information

- [Internal slack thread 1](https://dbt-labs.slack.com/archives/G01GRE7RQSF/p1775464249090249?thread_ts=1775455585.403859&cid=G01GRE7RQSF)
- [docs on unit tests for versioned models](https://docs.getdbt.com/reference/resource-properties/unit-testing-versions?version=1.12)
- [Internal slack thread 2](https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1775197563666909)