feat(provider): add TokenLab requester

[hedging8563]

Summary

• add TokenLab as a built-in LLM API requester
• reuse the existing LiteLLM/OpenAI-compatible requester path with https://api.tokenlab.sh/v1 as the default base URL
• include TokenLab search aliases for GPT, Claude, Gemini, DeepSeek, Qwen, Kimi, MiniMax, GLM, and Grok model families

Validation

• git diff --check
• parsed src/langbot/pkg/provider/modelmgr/requesters/tokenlabchatcmpl.yaml with Python/PyYAML

TokenLab docs: https://docs.tokenlab.sh/

[github-actions]

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