Send xunit_url only together with the overall result

The API requires result.overall whenever a request update contains result. TestingFarmRequest.update() added result.xunit_url whenever it got artifacts_url and test results existed, even without overall_result.

The pipeline stage reporter from ed5bb04b sends artifacts_url with every stage update. Once results exist, those updates carry xunit_url alone, and the API rejects them:

{"detail":[{"loc":["body","result","overall"],"msg":"field required","type":"value_error.missing"}]}

Add xunit_url only when the caller passes overall_result. The state reporter passes both in its final update, so the request still gets the xunit URL.

Generated-by: Claude Code