Additive, backwards-compatible test-double extensions so the discovery
rebuild failure path is observable:
- CreateFinderHook: optional callback invoked on every CreateFinder,
letting a test throw on the Nth call to simulate a runtime that
refuses to build a replacement finder.
- FinderCreatedCount: how many finders were built.
- The fake find handle now flips a Disposed flag, and GetCurrentSources
throws ObjectDisposedException if asked to read a disposed finder —
so a regression that polls a retired finder fails loudly instead of
silently passing (the fake previously ignored the handle entirely).
Default behavior is unchanged when CreateFinderHook is null.
