fix: 未知 Grok 文本兜底排除 image/video/audio 等媒体族

带版本号的媒体 ID（grok-2-image-1212、grok-5-video）会被
grok-N* 白名单当成文本，错吃 4.5 token 价。vision 多模态
仍按 token 计；HasIdentifiedTokenPricing 也不放行这些 ID。

backend/internal/service/billing_service.go

@@ -862,6 +862,9 @@ func (s *BillingService) grokUnknownTextFamilyFallback(model string) *ModelPrici
 
 func isGrokUnknownTextFamilyModel(model string) bool {
 	native := strings.ToLower(strings.TrimSpace(xai.StripGrokProviderPrefix(model)))
+	if isGrokMediaFamilyModel(native) {
+		return false
+	}
 	switch {
 	case native == "grok", native == "grok-latest":
 		return true
@@ -877,6 +880,19 @@ func isGrokUnknownTextFamilyModel(model string) bool {
 	}
 }
 
+// isGrokMediaFamilyModel matches ids that are billed per image/video/audio unit
+// rather than per token, so version-numbered media ids (grok-2-image-1212,
+// grok-5-video) cannot slip into the unknown-text fallback and pick up a token
+// card. "vision" is deliberately absent: multimodal chat models are token billed.
+func isGrokMediaFamilyModel(native string) bool {
+	for _, marker := range []string{"imagine", "image", "video", "audio", "speech", "tts", "transcribe", "realtime"} {
+		if strings.Contains(native, marker) {
+			return true
+		}
+	}
+	return false
+}
+
 // HasIdentifiedTokenPricing 判断模型能否在价格表中被"确定性识别"出 token 价格。
 //
 // 与 GetModelPricing 的关键区别：本函数拒绝按子串猜系列的兜底。GetModelPricing 会

backend/internal/service/billing_service_test.go

@@ -1221,6 +1221,14 @@ func TestGetModelPricing_UnknownGrokTextFallsBackToGrok45(t *testing.T) {
 		require.InDelta(t, baseline.CacheReadPricePerToken, pricing.CacheReadPricePerToken, 1e-12, model)
 	}
 
+	// Per-unit media ids must not inherit the text card just because they carry
+	// a version number; they are billed by the image/video/audio paths instead.
+	for _, model := range []string{"grok-2-image-1212", "grok-2-audio", "grok-5-video", "x-ai/grok-6-image"} {
+		require.False(t, isGrokUnknownTextFamilyModel(model), "model %s", model)
+	}
+	// Multimodal chat models stay token billed.
+	require.True(t, isGrokUnknownTextFamilyModel("grok-2-vision-1212"))
+
 	for _, model := range []string{
 		"grok-imagine-image-3.0",
 		"grok-imagine-video-2",

backend/internal/service/response_model_billing_test.go

@@ -372,6 +372,12 @@ func TestBillingServiceHasIdentifiedTokenPricing_RejectsFamilyGuesses(t *testing
 	}
 	require.False(t, billing.HasIdentifiedTokenPricing(""))
 	require.False(t, billing.HasIdentifiedTokenPricing("zz-unpriced-response-model"))
+	// Versioned media ids may inherit a text card via GetModelPricing; the
+	// identified-token gate must still reject them so response-model billing
+	// cannot adopt grok-4.5 rates for image/audio/video ids.
+	require.False(t, billing.HasIdentifiedTokenPricing("grok-2-image-1212"))
+	require.False(t, billing.HasIdentifiedTokenPricing("grok-2-audio"))
+	require.False(t, billing.HasIdentifiedTokenPricing("grok-5-video"))
 }
 
 func TestGatewayServiceRecordUsage_ResponseModelRejectsUnidentifiedFamilyName(t *testing.T) {