This commit addresses issue #21825, where invalid PERCENTILE values for
the `speculative_retry` setting were not properly handled, causing potential
server crashes. The valid range for PERCENTILE is between 0 and 100, as defined
in the documentation for speculative retry options, where values above 100 or
below 0 are invalid and should be rejected.
The added validation ensures that such invalid values are rejected with a clear
error message, improving system stability and user experience.
Fixes#21825Closesscylladb/scylladb#21879
69a0431cce