consolidate test runner rules to a shared rule, deprecate ios_test_runner

[aaronsky]

Still need to verify this thoroughly. Lots of things changed and I'm sure I missed a spot.

Does not remove ios_test_runner yet, so as to minimize the already myriad breaking changes inherent in this changeset.

Summary

• Adds a unified apple_xctestrun_runner for all Apple platform test targets.
  • Consolidates platform-specific runner scripts and xctestrun templates into shared implementations.
  • Derives the runner platform from the consuming test rule through an Apple configuration transition.
• Adds platform-aware simulator runtime and default-device selection.
• Preserves existing platform-specific runner APIs as compatibility macros.
• Deprecates ios_test_runner and the platform-specific runner APIs while retaining compatibility.

Breaking Changes

• Custom runner targets are now configured using the consuming test rule’s Apple transition.
• Direct references to removed platform-specific runner template files must migrate to the shared apple_xctestrun_runner templates.
• Tooling that inspects platform-specific runner rule kinds will now observe compatibility macros producing apple_xctestrun_runner targets.