[Fix] ratelimit: Track all buckets in selector rules

[Fix] ratelimit: Track all buckets in selector rules

When a rule defines several buckets (e.g. "200 / 1h" plus "30 / 1m")
and uses a selector, limit_to_prefixes keyed the prefixes table by the
selector value alone. Every bucket therefore mapped to the same Redis
key and only the last bucket in the array was ever tracked, so the
other limits were silently ignored.

Give each bucket a distinct key by prefixing the selector value with a
per-bucket id (burst + rate), mirroring the burst component that
gen_rate_key already prepends for the non-selector path. The
non-selector path is unchanged, so its existing Redis keys are kept.

Adds a functional regression test (two buckets, burst 2 + burst 20)
that fails before the fix because the restrictive bucket is ignored.

Closes #6059