Surface xctest runner errors as a dedicated Errors section on test runs

xctest runner failures (e.g. .xctest bundle could not be loaded, app could not launch) are no longer masquerading as test cases. The processor lifts them out of the test list into a first-class Errors concept, one entry per affected target with the real message (e.g. “Failed to create a bundle instance representing ‘…/.xctest’”). The test run page now shows a dedicated “Errors” card above the (renamed) “Test Failures” card, mirroring how Xcode surfaces them. Side effects for users: test counts no longer balloon with per-process synthetic cases, and the runaway test_case.created webhook fan-out that hit one project (~7,460 deliveries in 3 hours) is stopped at the source.