fix: resolve cherry-pick compilation and test issues

- Add int64(0) param to SelectAccountWithLoadAwareness callers (signature change from channel scheduling refactor)
- Add UsageMapHook type and struct field to StreamingProcessor
- Revert Claude Max cache billing code to upstream/main (not part of channel feature)
- Revert credits overages logic to upstream/main (non-channel change)
- Remove Instructions field reference (non-channel OpenAI feature)
- Restore sora_client_handler_test.go from upstream + add channel service nil params

backend/internal/service/antigravity_credits_overages_test.go

@@ -418,13 +418,7 @@ func TestShouldMarkCreditsExhausted(t *testing.T) {
 		require.True(t, shouldMarkCreditsExhausted(resp, body, nil))
 	})
 
-	t.Run("单模型配额耗尽不标记（积分对此无效）", func(t *testing.T) {
-		resp := &http.Response{StatusCode: http.StatusTooManyRequests}
-		body := []byte(`{"error":{"code":429,"message":"You have exhausted your capacity on this model. Your quota will reset after 146h11m17s.","status":"RESOURCE_EXHAUSTED"}}`)
-		require.False(t, shouldMarkCreditsExhausted(resp, body, nil))
-	})
-
-	t.Run("429 结构化限流也标记（积分注入后仍 429 即为耗尽）", func(t *testing.T) {
+	t.Run("结构化限流不标记", func(t *testing.T) {
 		resp := &http.Response{StatusCode: http.StatusTooManyRequests}
 		body := []byte(`{"error":{"status":"RESOURCE_EXHAUSTED","details":[{"@type":"type.googleapis.com/google.rpc.ErrorInfo","reason":"RATE_LIMIT_EXCEEDED"},{"@type":"type.googleapis.com/google.rpc.RetryInfo","retryDelay":"0.5s"}]}}`)
 		require.False(t, shouldMarkCreditsExhausted(resp, body, nil))