Stop the UI readiness helpers busy-waiting and cascading failures

[m96-chan]

Two defects in the helper #41 added, found reviewing it. #41 merged before this was pushed to the branch, so it comes as its own change.

waitUntilHittable was busy-waiting, not polling

_ = waitForExistence(timeout: 0.1)   // the 'throttle'

waitForExistence returns immediately when the element already exists — which is precisely the case this helper is written for: present but not yet touchable, a sheet still animating in. So the loop spun on back-to-back accessibility snapshot queries for the full twenty seconds instead of polling at 100ms.

Querying the app as hard as possible is a poor way to wait for it to settle, and one of the three failures this helper exists to prevent was Timed out while synthesizing event — the app failing to report itself idle. It sleeps now.

tapWhenReady recorded a failure and returned

continueAfterFailure is true by default, so the caller carried on. typeWhenReady would then type into nothing and raise a second, unrelated "no keyboard focus" error on top of the real one, and every later helper in the test would spend its own twenty seconds before doing the same — three times over, now that CI retries with -test-iterations 3.

Both helpers throw now, so a test stops at the first unmet precondition and the report is about what actually went wrong. Every call site takes try.

Same defect, one place older

TailscaleImportUITests.bringIntoView used waitForExistence(timeout: 0.2) as its throttle too, from before #41. Fixed alongside rather than left as the one place still doing it.

Verified

The whole UI suite on both idioms locally — iPhone 8 passing, iPad passing, simulators shut down after.

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