Probe remote cache availability during tuist bazel setup

Running tuist bazel setup now verifies that the resolved remote cache endpoint is actually reachable before writing .bazelrc.tuist. If the probe fails, setup aborts with a clear error, so users no longer get a generated Bazel config that only breaks at the first bazel build. The check covers the TUIST_CACHE_ENDPOINT override and single-server endpoint cases that previously skipped probing, and uses Bazel’s REAPI GetCapabilities handshake so connectivity, TLS, auth, and instance authorization are all validated.