fix: declare nltk as explicit dependency

nltk is required by granite citation/hallucination parsing
(nltk.sent_tokenize) but was only present as a transitive dependency
of rouge_score. Pin >=3.9 for punkt_tab support (security fix over
pickle-based punkt).

Author: planetf1

pyproject.toml

@@ -28,6 +28,7 @@ dependencies = [
     "mistletoe>=1.4.0",
     "pillow", # Needed for Intrinsics (HF and OpenAI Backends).
     "math_verify", # Needed for Majority Voting Sampling Strategies.
+    "nltk>=3.9", # Needed for sentence tokenization in granite citation parsing.
     "rouge_score", # Needed for Majority Voting Sampling Strategies.
     "PyYAML", # Needed for backends/adapters and granite formatters.
     "packaging", # Needed for server type helpers.