feat: support explicit prompt token tracking in SSE parsing and stream handlers

internal/sse/parser_test.go

@@ -19,6 +19,20 @@ func TestParseDeepSeekSSELineDone(t *testing.T) {
 	}
 }
 
+func TestExtractTokenUsage(t *testing.T) {
+	chunk := map[string]any{
+		"p": "response/token_usage",
+		"v": map[string]any{
+			"prompt_tokens":     123,
+			"completion_tokens": 456,
+		},
+	}
+	p, c := extractAccumulatedTokenUsage(chunk)
+	if p != 123 || c != 456 {
+		t.Fatalf("expected 123/456, got %d/%d", p, c)
+	}
+}
+
 func TestParseSSEChunkForContentSimple(t *testing.T) {
 	parts, finished, _ := ParseSSEChunkForContent(map[string]any{"v": "hello"}, false, "text")
 	if finished {