Add comprehensive GBNF grammar test suite

Validates the generated GBNF grammar for correctness:
syntax validation, completeness check, override coverage,
EBNF-to-GBNF rule coverage, and freshness verification.
Integrated into CI alongside existing EBNF validation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: reid-spencer

.github/workflows/scala.yml

@@ -198,6 +198,11 @@ jobs:
         cd language/jvm/src/test/python
         python ebnf_to_gbnf.py --check
 
+    - name: Validate GBNF Grammar
+      run: |
+        cd language/jvm/src/test/python
+        python gbnf_validator.py --skip-freshness
+
   dependency-check:
     timeout-minutes: 15
     permissions: