rust-driver: an undetermined RegionError is classified as a retryable cluster event #59

Found by the adversarial review of #57 (pin 3408d95). Latent: no scenario or driver op uses raw CAS today.

Since tikv/client-rust#555, raw compare_and_swap is terminal on an ambiguous outcome (src/raw/client.rs:704-706 at 3408d95): it returns the raw Error::RegionError carrying undetermined_result, or a gRPC error, and leaves the caller to classify it ("commit maps it to UndeterminedError").

crates/rust-driver/src/mapping.rs:135 maps every Error::RegionError(_) to Class::RegionError — an inadmissible cluster event the runner retries. So an undetermined CAS outcome, which is a fact about the operation (client rule WR-3: unknown is not failure, and it is not a blip), would be hidden as cluster noise instead of projected as Undetermined.

Fix: in classify_inner, a RegionError whose undetermined_result is set maps to Class::Undetermined (keeping the proto as evidence), before the generic RegionError arm; add a mapping unit test. Do it with the first CAS scenario at the latest, and check the Go driver projects client-go's undetermined CAS the same way.