fix: 兼容上游 SSE data: 后无空格格式，流式记账不再 0 消耗

火山方舟等上游流式返回 data:{...}（data: 后无空格），原解析只认
'data: '（带空格），导致 content 文本与 usage 全部漏解析：
- sseContentText 抽成 sseDataPayload，统一处理 data: 前缀
  （支持带/不带空格 + event:+data: 多行块）
- scanUsage 复用 sseDataPayload
- convert/stream.go parseLine 也支持 data:{ 无空格
- 新增回归测试覆盖无空格/带空格/多行块三种形态

server/internal/proxy/sse_fix_test.go

@@ -0,0 +1,48 @@
+package proxy
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// 复现线上火山方舟 qwen 流：data:{...} 无空格（省略 data: 后的空格）。
+func TestSSEDataPayloadNoSpace(t *testing.T) {
+	lines := []string{
+		`data:{"message":{"model":"qwen3.8-flash","id":"msg_1","role":"assistant","type":"message","content":[],"usage":{"input_tokens":31626,"output_tokens":0}},"type":"message_start"}`,
+		`data:{"delta":{"type":"text_delta","text":"你好"},"type":"content_block_delta","index":0}`,
+		`data:{"delta":{"type":"text_delta","text":"!"},"type":"content_block_delta","index":0}`,
+		`data:{"delta":{"stop_reason":"end_turn"},"type":"message_delta","usage":{"cache_creation":{"ephemeral_5m_input_tokens":33065},"output_tokens":8,"cache_creation_input_tokens":33065,"input_tokens":8,"cache_read_input_tokens":0}}`,
+	}
+	var out string
+	for _, l := range lines {
+		out += sseContentText([]byte(l))
+	}
+	if out != "你好!" {
+		t.Fatalf("outputText=%q, want %q", out, "你好!")
+	}
+	// message_delta 的 delta.usage 应能提取（output_tokens=8）
+	u := scanUsage([]byte(lines[3]))
+	if u == nil {
+		t.Fatal("scanUsage returned nil for message_delta with usage")
+	}
+	var sh usageShape
+	if err := json.Unmarshal(u, &sh); err != nil {
+		t.Fatalf("unmarshal usage: %v", err)
+	}
+	if sh.OutputTokens != 8 {
+		t.Fatalf("output_tokens=%d, want 8", sh.OutputTokens)
+	}
+}
+
+// 兼容带空格的单 data: 行（标准 SSE）与 event:+data: 多行块。
+func TestSSEDataPayloadSpacedAndMultiLine(t *testing.T) {
+	// 标准：data: {...}
+	if got := sseContentText([]byte(`data: {"delta":{"type":"text_delta","text":"hi"},"type":"content_block_delta","index":0}`)); got != "hi" {
+		t.Fatalf("spaced single line: got %q, want hi", got)
+	}
+	// 多行块：event: message_delta\ndata: {...}
+	block := []byte("event: message_delta\ndata: {\"delta\":{\"type\":\"text_delta\",\"text\":\"yo\"},\"type\":\"content_block_delta\",\"index\":0}\n")
+	if got := sseContentText(block); got != "yo" {
+		t.Fatalf("multiline block: got %q, want yo", got)
+	}
+}