Add support for Grok 4.20 beta (#5144)

It matches the config for Grok 4 almost exactly.

Sourced from commit da722d4ef71c52a137bd3c112ed7949272f81bd2

Author: rgambee

src/futuresearch/generated/models/llm_enum_public.py

@@ -57,6 +57,7 @@ class LLMEnumPublic(str, Enum):
     GPT_5_MINI = "GPT_5_MINI"
     GPT_5_MINIMAL = "GPT_5_MINIMAL"
     GPT_5_NANO = "GPT_5_NANO"
+    GROK_4_20_BETA = "GROK_4_20_BETA"
 
     def __str__(self) -> str:
         return str(self.value)