### What problem does this PR solve?
Implement OpenAI chat completions in GO
POST /api/v1/openai/<chat_id>/chat/completions
OpenAI chat cli: internal/development.md
### Type of change
- [x] Refactoring
## Summary
- Add knowledge-base retrieval support to Go chat completions.
## What changed
- Routes KB-backed chat sessions through the Go retrieval service
instead of falling back to solo chat.
- Resolves embedding and rerank models, validates accessible knowledge
bases, and preserves tenant-aware retrieval.
- Rejects mixed embedding models across selected knowledge bases before
retrieval to avoid incompatible vector dimensions.
- Threads the HTTP request context into streaming retrieval so cancelled
requests can stop downstream retrieval work.
- Applies metadata filters and message-level `doc_ids` before retrieval.
- Expands parent/child chunks before building references and prompt
context.
- Injects retrieved knowledge through a copied dialog prompt config so
the caller's original dialog is not mutated.
- Honors configured empty responses when no chunks are found.
- Names the metadata no-match sentinel and reuses it across
retrieval/handler paths.
- Adds a defensive content cast while appending streamed answers.
- Adds focused unit coverage for retrieval, metadata filtering,
authorization, multimodal messages, references, empty-response behavior,
prompt immutability, and mixed embedding models.
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Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
