feat: add MiniMax provider support (#9095)

* feat: add MiniMax provider support

Add MiniMax as a new LLM provider using their Anthropic-compatible API.

Key implementation details:
- Uses standard messages API (not beta) - MiniMax supports thinking blocks natively
- Base URL: https://api.minimax.io/anthropic
- Models: MiniMax-M2.1, MiniMax-M2.1-lightning, MiniMax-M2 (all 200K context, 128K output)
- Temperature clamped to valid range (0.0, 1.0]
- All M2.x models treated as reasoning models (support interleaved thinking)

Files added:
- letta/schemas/providers/minimax.py - MiniMax provider schema
- letta/llm_api/minimax_client.py - Client extending AnthropicClient
- tests/test_minimax_client.py - Unit tests (13 tests)
- tests/model_settings/minimax-m2.1.json - Integration test config

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* chore: regenerate API spec with MiniMax provider

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* chore: use MiniMax-M2.1-lightning for CI tests

Switch to the faster/cheaper lightning model variant for integration tests.

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* chore: add MINIMAX_API_KEY to deploy-core command

Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

* chore: regenerate web openapi spec with MiniMax provider

Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

🐾 Generated with [Letta Code](https://letta.com)

---------

Co-authored-by: Letta <noreply@letta.com>
Co-authored-by: letta-code <248085862+letta-code@users.noreply.github.com>
Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

fern/openapi.json

@@ -35719,6 +35719,7 @@
               "koboldcpp",
               "vllm",
               "hugging-face",
+              "minimax",
               "mistral",
               "together",
               "bedrock",
@@ -38206,6 +38207,7 @@
               "koboldcpp",
               "vllm",
               "hugging-face",
+              "minimax",
               "mistral",
               "together",
               "bedrock",
@@ -39822,6 +39824,7 @@
           "hugging-face",
           "letta",
           "lmstudio_openai",
+          "minimax",
           "mistral",
           "ollama",
           "openai",