Fixed concurrent collection access crash

[rodgers-r]

The way the segment event handlers were being dispatched caused a rare exception if a new handler was added/removed by ProcessAsync:

Unhandled exception. System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at Whisper.net.WhisperProcessor.OnNewSegment(IntPtr state)
at Whisper.net.WhisperProcessor.OnNewSegmentStatic(IntPtr ctx, IntPtr state, Int32 nNew, IntPtr userData)

This patch fixes the crash by preventing concurrent access to the collection.

[sandrohanea]

Hello @rodgers-r ,
Indeed, it seems that the bug was there and also appeared in the past in the pipelines: https://github.com/sandrohanea/whisper.net/actions/runs/17191846947/job/48768577897.

Thanks for the patch!

[sandrohanea]

The other pipeline issues are not related to the PR and will be fixed in another PR, merging now.