Recover from duplicate accessibility deferral receivers

[robmaceachern]

Summary

Fixes the duplicate accessibility deferral receiver recovery path so it clears deferred content without triggering a DEBUG assertion. This matches the existing recovery behavior while avoiding the crash seen when integrating Blueprint 6.7.2 downstream.

Also wires BlueprintUIAccessibilityCoreTests into Package.swift, so the accessibility core regression tests run through the package scheme.

Root Cause

AccessibilityDeferral.ParentContainer already had recovery logic for multiple receivers, but it called assertionFailure first. In DEBUG builds, that turned the recoverable duplicate-receiver state into a fatal test crash for consumers such as squareup/ios-register#153221.

Validation

• Reproduced the failure before the fix with:
  xcodebuild test -scheme BlueprintUI-Package -destination 'id=C4028877-CBF8-4816-88AD-B9C88E490FB1' -only-testing:BlueprintUIAccessibilityCoreTests/AccessibilityDeferralTests/test_parentContainerWithDuplicateReceiversClearsDeferralContent
• Passed after the fix:
  xcodebuild test -scheme BlueprintUI-Package -destination 'id=C4028877-CBF8-4816-88AD-B9C88E490FB1' -only-testing:BlueprintUIAccessibilityCoreTests
• Passed:
  mise x -- swiftformat --lint .

I also ran the full BlueprintUI-Package suite locally. The new accessibility core target and main Blueprint tests passed, but the full run still exited failing on unrelated BlueprintUICommonControlsTests snapshot reference gaps for local iOS 18 output and one existing measurement expectation.

[RoyalPineapple]

Confirmed multiple Receivers in one deferral container is a valid, intended configuration e.g. a card field whose number/expiry/CVV/ZIP subfields are each receivers, so the container should recover gracefully instead of asserting.