Fix uncaught exceptions in IndexIVF::search_preassigned OpenMP region (#5037)

Summary:
Pull Request resolved: #5037

C++ exceptions thrown inside `#pragma omp parallel` regions that are not
caught within the region call `std::terminate` — they cannot propagate
across thread boundaries.

`IndexIVF::search_preassigned` had multiple unguarded throw sites inside
its OpenMP parallel region:

1. **`scan_one_list` lambda**:
   `FAISS_THROW_IF_NOT_FMT(key < nlist)` and `scanner->set_list()` were
   above the try-catch block, so a corrupt key >= nlist would throw
   uncaught and call `std::terminate`.

2. **Outer parallel region**:
   `get_InvertedListScanner()` and `scanner->set_query()` had no
   try-catch at all.

Fixes:

1. Moved the existing try-catch in `scan_one_list` up to also cover the
   key validation and `set_list` calls.

2. Wrapped the entire `#pragma omp parallel` body in a try-catch that
   uses the existing `interrupt`/`exception_string`/`exception_mutex`
   pattern to safely propagate exceptions out of the parallel region.

Reviewed By: mnorris11

Differential Revision: D99455250

fbshipit-source-id: 41e8816d71c031b1e8526f76bd5a8936f4d4f160

Author: scsiguy