Add Denizen Blu as an LLM provider

[djrok212]

Summary

Adds support for Denizen Blu, an OpenAI-compatible model gateway, as a new default provider.

• core/llm/llms/DenizenBlu.ts — subclasses OpenAI (fixed-endpoint OpenAI-compatible provider, same pattern as Groq/DeepInfra)
• Registered in core/llm/llms/index.ts
• Added to PROVIDER_SUPPORTS_IMAGES in core/llm/autodetect.ts
• Docs page at docs/customize/model-providers/more/denizenblu.mdx with config.yaml examples

Test plan

• tsc --noEmit passes clean across core/
• Prettier formatting passes on all changed/new files
• Manually tested in the VS Code extension debug host with a live Denizen Blu API key — confirmed chat + streaming work end-to-end with both qwen2.5-coder-1.5b-instruct and qwen3-0.6b

Screen recording of it working attached below.

Screen.Recording.2026-08-18.at.12.19.57.PM.mov

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[djrok212]

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