fix: exclude Rust messages rollout flag from env docs check

Co-Authored-By: Ishaan Jaffer <155045088+ishaan-berri@users.noreply.github.com>

tests/documentation_tests/test_env_keys.py

@@ -27,6 +27,7 @@ EXCLUDED_GUARD_ONLY_VARS = {
 # Temporary/internal rollout flags are intentionally not added to the public
 # environment settings docs until the feature is ready for broad use.
 EXCLUDED_ROLLOUT_FLAGS = {
+    "LITELLM_USE_RUST_MESSAGES",
     "LITELLM_USE_RUST_OCR",
 }