feat: add OpenAI GPT-5.1 series models

- add support for gpt-5.1, gpt-5.1-2025-11-13, gpt-5.1-chat-latest, gpt-5.1-codex, gpt-5.1-codex-mini
- replace default model gpt-5-2025-08-07 with gpt-5.1-2025-11-13 per OpenAI’s Nov 13 launch

See: https://openai.com/index/gpt-5-1-for-developers/

Author: PeterDaveHello

pr_agent/algo/__init__.py

@@ -32,6 +32,11 @@
     'gpt-5-mini': 200000,  # 200K, but may be limited by config.max_model_tokens
     'gpt-5': 200000,
     'gpt-5-2025-08-07': 200000,
+    'gpt-5.1': 200000,
+    'gpt-5.1-2025-11-13': 200000,
+    'gpt-5.1-chat-latest': 200000,
+    'gpt-5.1-codex': 200000,
+    'gpt-5.1-codex-mini': 200000,
     'o1-mini': 128000,  # 128K, but may be limited by config.max_model_tokens
     'o1-mini-2024-09-12': 128000,  # 128K, but may be limited by config.max_model_tokens
     'o1-preview': 128000,  # 128K, but may be limited by config.max_model_tokens