Consolidate xctestrun runners across Apple platforms

[aaronsky]

Recreation of #3008 as a stack

• Add a shared apple_xctestrun_runner for iOS, macOS, tvOS, visionOS, and watchOS tests.
• Consolidate platform-specific shell and xctestrun templates into shared implementations with distinct execution phases.
• Derive the test platform from the consuming rule's transitioned Apple configuration.
• Migrate the platform default runners and retain the previous public runner names as compatibility macros.
• Consolidate runner integration tests and move the shared rule documentation to rules-apple.md.

Stack 3 of 4. Depends on #3060 and is followed by #3062.

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