fix(tests): correct validation key from "test" to "tests" in robot helper

The verify_validation_sub_fields() helper checked for key "test" but
_apply_output_dict() uses "tests" (plural), causing the Robot Framework
integration test "Validation Contains Test Lint Type Check" to always
exit with rc=1.

ISSUES CLOSED: #9449

robot/helper_plan_apply_json_envelope.py

@@ -148,7 +148,7 @@ def verify_validation_sub_fields():
     """Verify data.validation has test, lint, type_check."""
     env = _envelope()
     v = env.get("data", {}).get("validation", {})
-    req = {"test", "lint", "type_check"}
+    req = {"tests", "lint", "type_check"}
     if req <= set(v.keys()):
         _ok(f"validation keys={sorted(v.keys())}")
     _err(f"Missing: {req - set(v.keys())}")