Add gemini-3.1-pro-preview model to reasoning models (OpenHands#2270)

Cherry-pick from upstream 0989fb2

Author: zparnold

openhands-sdk/openhands/sdk/llm/utils/model_features.py

@@ -66,6 +66,7 @@ class ModelFeatures:
     # Gemini 3 family
     "gemini-3-flash-preview",
     "gemini-3-pro-preview",
+    "gemini-3.1-pro-preview",
     # OpenAI GPT-5 family (includes mini variants)
     "gpt-5",
     # Anthropic Opus 4.5 and 4.6

tests/sdk/llm/test_model_features.py

@@ -35,6 +35,7 @@ def test_model_matches(name, pattern, expected):
         # Gemini 3 family
         ("gemini-3-pro-preview", True),
         ("gemini-3-flash-preview", True),
+        ("gemini-3.1-pro-preview", True),
         # GPT-5 family
         ("gpt-5.2", True),
         ("gpt-5.2-codex", True),