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/shared/upstream_empty.go

@@ -1,9 +1,12 @@
 package shared
 
-import "net/http"
+import (
+	"net/http"
+	"strings"
+)
 
-func ShouldWriteUpstreamEmptyOutputError(text string) bool {
-	return text == ""
+func ShouldWriteUpstreamEmptyOutputError(text, thinking string) bool {
+	return strings.TrimSpace(text) == "" && strings.TrimSpace(thinking) == ""
 }
 
 func UpstreamEmptyOutputDetail(contentFilter bool, text, thinking string) (int, string, string) {
@@ -18,7 +21,7 @@ func UpstreamEmptyOutputDetail(contentFilter bool, text, thinking string) (int,
 }
 
 func WriteUpstreamEmptyOutputError(w http.ResponseWriter, text, thinking string, contentFilter bool) bool {
-	if !ShouldWriteUpstreamEmptyOutputError(text) {
+	if !ShouldWriteUpstreamEmptyOutputError(text, thinking) {
 		return false
 	}
 	status, message, code := UpstreamEmptyOutputDetail(contentFilter, text, thinking)