Relate to #17284.
## Summary
Batch 5/6 migrated the rest of the Go model drivers onto the shared HTTP
helpers (`doRequest`, `doStreamRequest`, `applyAuth`). This PR completes
the batch for the remaining OpenAI-compatible chat-streaming drivers
that were still hand-writing HTTP requests:
- **7 drop-in migrations**: deepseek, gpustack, groq, longcat, moonshot,
openai, siliconflow
- **1 adapter migration**: minimax (relocated its `io.Pipe`
error-interception into the `doStreamRequest` handler)
- **1 full migration**: azure_openai (all four paths: chat, streaming,
embeddings, list-models) plus the auth header hook
- **1 receiver fix**: nvidia `NewInstance` value → pointer
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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>