fix(test): update FilterThinkingBlocksForRetry call to use mappedModel param

The function signature was changed to require a mappedModel parameter
for protocol-aware thinking-block filtering, but this test call site
was not updated. Without the fix, the unit test build fails on CI.

backend/internal/service/gateway_request_test.go

@@ -401,7 +401,7 @@ func TestFilterThinkingBlocksForRetry_DropsThinkingBlockWithEmptyContent(t *test
 		]
 	}`)
 
-	out := FilterThinkingBlocksForRetry(input)
+	out := FilterThinkingBlocksForRetry(input, "claude-sonnet-4-5")
 
 	var req map[string]any
 	require.NoError(t, json.Unmarshal(out, &req))