feat(go-api): add RAG retrieval to chat completions (#15739)

## 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.

---------

Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

internal/handler/chat_session.go

@@ -292,9 +292,10 @@ func (h *ChatSessionHandler) Completion(c *gin.Context) {
 
 		// Create a channel for streaming data
 		streamChan := make(chan string)
+		reqCtx := c.Request.Context()
 		go func() {
 			defer close(streamChan)
-			err := h.chatSessionService.CompletionStream(userID, req.ConversationID, processedMessages, req.LLMID, chatModelConfig, messageID, streamChan)
+			err := h.chatSessionService.CompletionStream(reqCtx, userID, req.ConversationID, processedMessages, req.LLMID, chatModelConfig, messageID, streamChan)
 			if err != nil {
 				streamChan <- fmt.Sprintf("data: %s\n\n", err.Error())
 			}