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_toolcall_test.go

@@ -133,13 +133,18 @@ func TestHandleNonStreamReturns429WhenUpstreamHasOnlyThinking(t *testing.T) {
 	rec := httptest.NewRecorder()
 
 	h.handleNonStream(rec, resp, "cid-thinking-only", "deepseek-v4-pro", "prompt", 0, true, false, nil, nil, nil)
-	if rec.Code != http.StatusTooManyRequests {
-		t.Fatalf("expected status 429 for thinking-only upstream output, got %d body=%s", rec.Code, rec.Body.String())
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected status 200 for thinking-only upstream output, got %d body=%s", rec.Code, rec.Body.String())
 	}
 	out := decodeJSONBody(t, rec.Body.String())
-	errObj, _ := out["error"].(map[string]any)
-	if asString(errObj["code"]) != "upstream_empty_output" {
-		t.Fatalf("expected code=upstream_empty_output, got %#v", out)
+	choices, _ := out["choices"].([]any)
+	if len(choices) == 0 {
+		t.Fatal("expected at least one choice")
+	}
+	first, _ := choices[0].(map[string]any)
+	msg, _ := first["message"].(map[string]any)
+	if asString(msg["reasoning_content"]) != "Only thinking" {
+		t.Fatalf("expected reasoning_content='Only thinking', got %#v", msg)
 	}
 }