Add context windows for grok-4 models (#3043)

Author: runtimeBob

letta/schemas/providers/xai.py

@@ -16,6 +16,9 @@
     "grok-3-mini": 131_072,
     "grok-3-mini-fast": 131_072,
     "grok-4-0709": 256_000,
+    "grok-4-fast-reasoning": 2_000_000,
+    "grok-4-fast-non-reasoning": 2_000_000,
+    "grok-code-fast-1": 256_000
 }