Synchronize CanGetInstance with delegate creation

[seesharper]

Fixes #614

Problem

CreateDelegate holds lockObject while building emit methods, but CanGetInstance walks the same GetEmitMethod → CreateEmitMethodForUnknownService path without the lock. That path can register closed generic services, appending to the List<ServiceRegistration> / List<EmitMethodInfo> values in allRegistrations / allEmitters while another thread enumerates them in CreateEmitMethodForEnumerableServiceServiceRequest:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Linq.Enumerable+SelectManySingleSelectorIterator...
   at LightInject.ServiceContainer.CreateEmitMethodForEnumerableServiceServiceRequest(...)

With LightInject.Microsoft.DependencyInjection, CanGetInstance backs IServiceProviderIsService, which ActivatorUtilities (Hangfire, MVC, etc.) calls at runtime, so this shows up under production load.

Fix

Take lockObject in CanGetInstance. Monitor is reentrant, so nested calls from within CreateDelegate are unaffected.

Tests

Issue614 runs CanGetInstance and GetInstance concurrently for many open generic enumerables with MS compatibility enabled. It fails consistently on master (corrupted lists from concurrent List.Add) and passes with the fix. Full suite passes (1562 tests).

A separate, pre-existing issue where closed generic enumerables accumulate duplicate items in MS compatibility mode will be addressed in a follow-up PR.

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