Replace abandoned fakeredis with real redis, upgrade redis-rb to 5.x

[dduugg]

What & why

fakeredis is unmaintained — its last release (0.9.2, May 2023) hard-pins redis ~> 4.8, which is why every Dependabot bump (including #19) left redis stuck at 4.8.1 while it moved everything else. There is no fakeredis release that supports redis-rb 5, so this replaces it rather than upgrading it.

The specs never exercised fakeredis's in-memory data store — they only assert connection types and call verify!/quit. So this switches the test suite to a real redis server, which is what the gem actually ships against.

Changes

• Remove fakeredis from the Gemfile and delete the spec/support/fakeredis.rb global stub
• Relock: redis 4.8.1 → 5.4.1 (pulls in redis-client)
• Add a redis service to the CI test_and_lint job — fakeredis was previously the only reason specs passed without a server

Notes

• The gemspec still declares redis ">= 4", so downstream apps on redis 4 are not forced to upgrade — this only advances the dev/test lock.
• Specs required no rewrite: redis-rb 5 connects lazily, so type assertions and verify!/quit work against the live server.

Verification

• bundle exec rake → 10 examples, 0 failures (run against a local redis)
• bundle exec standardrb → clean