grammar: add test case for hang in repetition grammar processing

This commit adds a new test case to the grammar integration tests that
specifically targets a hang scenario in the repetition grammar parser
found while adding GBNF support to ripgrep-edit.

llama-server reproducer:

curl \
  -X POST \
  -d '{
    "messages": [{ "role": "user", "content": "write yes" }],
    "grammar": "root ::= (([^x]*){0,99}){0,99}"
  }' \
  -H "Content-Type: application/json" \
  http://localhost:8811/v1/chat/completions

Not security related according to
https://github.com/ggml-org/llama.cpp/security#untrusted-environments-or-networks

tests/test-grammar-integration.cpp

@@ -802,6 +802,19 @@ static void test_quantifiers() {
 		"yy"
         }
     );
+    test_grammar(
+        "hang",
+        // Grammar
+        R"""(
+            root ::= (((((([^x]*){0,99}){0,99}){0,99}){0,99}){0,99}){0,99}
+        )""",
+        // Passing strings
+        {
+        },
+        // Failing strings
+        {
+        }
+    );
 }
 
 static void test_failure_missing_root() {