fix: Unpin litellm upper bound to allow CVE-2026-35030 remediation

FUTURE_COPYBARA_INTEGRATE_REVIEW=#6596 from googleapis:release-please--branches--main b82c8bd
PiperOrigin-RevId: 902906101

Author: speedstorm1

setup.py

@@ -181,8 +181,8 @@
     "jsonschema",
     "ruamel.yaml",
     "pyyaml",
-    "litellm>=1.75.5, <=1.82.6",
-    # For LiteLLM tests. Upper bound pinned: versions 1.82.7+ compromised in supply chain attack.
+    "litellm>=1.75.5, !=1.82.7, !=1.82.8",
+    # For LiteLLM tests. Exclude compromised versions 1.82.7 and 1.82.8 (supply chain attack).
 ]
 
 langchain_extra_require = [