ci: validate matrix patches before release (scylla-3.x)

[nikagra]

Closes scylladb/scylla-java-driver-matrix#145

What

Adds a "Validate matrix patches" step to the release workflow, inserted between Prepare release (which creates the tag locally via mvn release:prepare) and Perform release (which publishes to Maven Central).

The step clones scylla-java-driver-matrix at the specified branch and runs --patch-only mode against the newly created local tag. If patches fail to apply, the release workflow aborts before anything is published or pushed.

Why

Today, a broken or missing matrix patch is only discovered after the tag is on GitHub and the artifact is on Maven Central — too late for a clean abort. This change makes patch compatibility a hard gate before publication.

Changes

.github/workflows/release.yml:

• Add matrix-branch input (type: string, default: master) — allows pointing to a non-merged matrix branch if patches need updating before the matrix PR merges
• Add "Validate matrix patches" step after "Prepare release":
  1. Extracts the release tag from release.properties (scm.tag=X.Y.Z.W)
  2. Clones scylladb/scylla-java-driver-matrix at the specified branch
  3. Runs python3 main.py . --versions {tag} --driver-type scylla --patch-only
  4. Fails the workflow if patches don't apply

Release flow with this change

1. mvn release:prepare          → creates tag locally (e.g., 3.11.5.16)
2. Validate matrix patches      → clone matrix, run --patch-only
   ├── patches apply cleanly    → continue
   └── patches FAIL             → release ABORTS (nothing published, no tag pushed)
3. mvn release:perform          → publish to Maven Central
4. git push --follow-tags       → push tag to GitHub

Testing

Use dry-run: true — the "Validate matrix patches" step has no if condition so it runs in both normal and dry-run mode. This allows testing the validation without publishing or pushing anything.

Dependency

Requires scylladb/scylla-java-driver-matrix#146 (--patch-only mode) to be merged into matrix master before this workflow runs in production. During the interim, the matrix-branch input can point to the feature branch.