## Summary

- Ports the `gradle-tests` job from main (#24714) into this branch's
`validation.yml`, keeping the branch's own Node version.
- Adds `_MERGE_GROUP_BASE_SHA` and `_BASE_REF` to the workflow env,
which the job's change detection reads.
- `functionalTest` now sets `vaadin.version` to the version being built,
so the generated sample applications resolve `com.vaadin:flow` from the
restored workspace instead of a published snapshot.
- `AbstractGradleTest` fails instead of falling back to the published
version when `CI` is set and the variable is missing.

## Context

The TeamCity `Gradle Tests` build has no VCS trigger any more, so the
`Gradle Tests (Flow Pull Requests Validation set)` context it publishes
never reports, and every pull request against this branch stays blocked
on a required check that cannot arrive. Once this is merged, branch
protection can require the `Gradle Tests` job instead and the TeamCity
context can be dropped.