Fix race condition in ModuleEngineRegistry with ReaderWriterLockSlim

[Copilot]

ModuleEngineRegistry had a race condition where concurrent Register() or Reset() calls could modify the Engines dictionary while GetEngines(), FindWebhookEngine(), or FindById() were iterating over it, potentially causing InvalidOperationException.

Changes

• Added ReaderWriterLockSlim to protect all registry operations
  • Write locks for Register() and Reset() (mutating operations)
  • Read locks for GetEngines(), FindWebhookEngine(), and FindById() (query operations)
  • Simplified GetEngines() to use single read lock instead of per-list locks

The read lock prevents all writes during enumeration, eliminating the race condition while maintaining efficient concurrent reads.

public static IReadOnlyCollection<IModuleEngineDescriptor> GetEngines()
{
    RegistryLock.EnterReadLock();
    try
    {
        // The read lock ensures no modifications occur during enumeration
        return Engines.Values.SelectMany(list => list).ToArray();
    }
    finally
    {
        RegistryLock.ExitReadLock();
    }
}

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