Add GPT-5.1 family support

[PeterDaveHello]

Summary from GitHub Copilot:

This pull request adds support for the new GPT-5.1 family of models throughout the codebase, including constants, configuration, and UI options. It also ensures that model alias resolution and context lengths are handled for these new models, and adds a test to verify alias resolution.

Support for GPT-5.1 models:

• Added new constants for GPT_5_1_CHAT_LATEST_MODEL, GPT_5_1_MODEL, GPT_5_1_2025_11_13_MODEL, GPT_5_1_CODEX_MODEL, and GPT_5_1_CODEX_MINI_MODEL in app/openai_constants.py.
• Registered these new models in the versioning, context length, and alias resolution mappings in app/openai_constants.py. [1] [2] [3]

Slack UI integration:

• Imported the new GPT-5.1 model constants in app/slack_ui.py and added them as selectable options in the model configuration modal. [1] [2]

Testing:

• Added a test in tests/model_constants_test.py to ensure that the GPT_5_1_MODEL alias resolves correctly to the dated release.