Add iteration-count accessors to SolvedModel

[GermanHeim]

After a solve, HiGHS exposes per-algorithm iteration counts through its solution-info values (simplex_iteration_count, ipm_iteration_count, etc.).
Right now, there's no safe way to read them: a caller has to reach for SolvedModel::as_mut_ptr() and call the raw Highs_getIntInfoValue FFI itself.

With this PR, we add five new public methods on SolvedModel, mirroring the existing accessors:
simplex_iteration_count(), ipm_iteration_count(), qp_iteration_count(), pdlp_iteration_count(), and crossover_iteration_count().

I also addeda small private int_info_value(name) helper that wraps Highs_getIntInfoValue + try_handle_status.