Remove XCTest observer on the main thread during framework unload

[ypopovych]

Problem

XCTestObservationCenter.removeTestObserver (like addTestObserver) is main-thread-only. The SDK's observer lifecycle is asymmetric:

• Added from the load-time C constructor __AutoLoadHook → libraryLoaded() → start() → addTestObserver. Constructors run at dylib load = main thread → fine.
• Removed from the unload-time C destructor __AutoUnloadHook → libraryUnloaded() → stop() → removeObserver() → removeTestObserver. Nothing hopped to the main thread.

The destructor's thread depends on who calls exit():

Runner	exit() caller	Thread	Result
Classic xctest (Xcode / iOS-sim)	after the run loop	main	✅ fine
swift-testing / swift test	Runner.main async continuation	Swift Concurrency background thread	💥 crash

So under the swift-testing runner, teardown raised:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException',
    reason: 'Test observers can only be registered and unregistered on the main thread.'
   -[XCTestObservationCenter removeTestObserver:]  ←  __AutoUnloadHook

Pre-existing; independent of any OpenTelemetry work.

Fix

removeObserver() now removes on the main thread — directly when already on it, otherwise via DispatchQueue.main.async. The hop is async on purpose: a synchronous hop can deadlock during process teardown if the main thread is already finalizing, and if the block never runs because the process exits first, that's harmless (the observer only needs detaching while the runner is alive).

🤖 Generated with Claude Code

[datadog-datadog-prod-us1]

More details

This PR fixes a critical crash in the Swift Testing runner by ensuring XCTest observer removal happens on the main thread. The SDK's observer lifecycle was asymmetric — added on main thread during dylib load, but removed during unload from a background thread under swift-testing, causing NSInternalInconsistencyException. The fix adds a thread check: direct removal on main thread, async dispatch to main queue from background threads. State transitions synchronously, preventing deadlocks during process teardown. The change is minimal, well-documented, and handles all three execution paths safely (main thread, background thread, process exit before async block runs).

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