[Apple] Handle iOS 18+ test results via file copy instead of TCP tunnel

[kotlarmilos]

Description

XHarness currently retrieves test results over a TCP tunnel between the device and host. On iOS/tvOS 18 and newer, tunnels no longer work. This PR updates the device test runner to write results to file and implements a new ResultFileHandler to copy them to the host.

Changes

• TestRunner is updated to write results to a file on devices and simulators with version >=18
• AppTester is updated to invoke ResultFileHandler after test execution
• ResultFileHandler is implemented to copy results file from device to host

Backwards and forward compatibility

Host can work with old app bundles (without the new test runner) on OS versions < 18. For versions >= 18, apps must be recompiled with the updated test runner to support file‑based result retrieval.

Validation

• Unit tests for ResultFileHandler are implemented.
• This PR was tested on both default and scouting queues: 9a2345a and 9a2345a

Testing device	Default version	Scouting version
iOS	16.5	18.1.1
tvOS	16.5	18.5
iOSSimulator	17.0	N/A
tvOSSimulator	17.0	18.0
MacCatalyst	N/A	N/A

Next steps

• Create tracking issue for mlaunch failures: [apple] mlaunch errors with ios/tvos devices macios#23038
• Re-build test bundles and run tests
• Confirm device, simulators, and maccatalyst tests are running successfully
• Release the new XHarness version
• Pull the new XHarness version in dotnet/runtime and run tests

Fixes #1367 dotnet/macios#22632

[kotlarmilos]

This PR is ready for review. It was tested on both default and scouting queues.