perf(streaming): optimize TTFT and reduce buffering latency

Core changes:
- stream.go: New accumulation buffer architecture with scanner goroutine
  + select loop, MinChars=16, MaxWait=10ms, first-flush-immediate
- dedupe.go: Add TrimContinuationOverlapFromBuilder to avoid string copies
- claude/stream_runtime_core.go: Integrate toolstream for incremental text
- claude/stream_runtime_finalize.go: toolstream flush support
- stream_emitter.js: Reduce DeltaCoalescer thresholds (160->16 chars, 80->20ms)
- empty_retry: Add thinking-aware empty output detection
- Fix reasoning_content leak and finish_reason=null in edge cases
- Fix tail content truncation when max_tokens exceeded

Tests: sync test expectations with upstream for thinking content

internal/httpapi/openai/chat/handler_chat.go

@@ -168,7 +168,7 @@ func (h *Handler) handleNonStream(w http.ResponseWriter, resp *http.Response, co
 		finalText = replaceCitationMarkersWithLinks(finalText, result.CitationLinks)
 	}
 	detected := detectAssistantToolCalls(result.Text, finalText, result.Thinking, result.ToolDetectionThinking, toolNames)
-	if shouldWriteUpstreamEmptyOutputError(finalText) && len(detected.Calls) == 0 {
+	if shouldWriteUpstreamEmptyOutputError(finalText, finalThinking) && len(detected.Calls) == 0 {
 		status, message, code := upstreamEmptyOutputDetail(result.ContentFilter, finalText, finalThinking)
 		if historySession != nil {
 			historySession.error(status, message, code, finalThinking, finalText)