…AI4Code#6+FSoft-AI4Code#2)

- Structured error classification (LLMError/PipelineError/CancellationError)
- Cancellable pipeline with CancellationToken + "cancelled" status
- LLM retry wrapper with streaming fallback (openai_compatible only)
- model_list polymorphic items: str or {name, stream}

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…de#6→FSoft-AI4Code#2)

7 tasks across 3 phases: error classification, cancellation, retry+streaming.
Fixed: model ref format, scheduler keeps existing checks, test fixtures, loader normalization.

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Co-Authored-By: HAPI <noreply@hapi.run>