fix: handle disabled cache backend features in multilevel chain

[cryptomilk]

When a backend feature (e.g. redis or s3) is not compiled in but another remote backend feature is, the match arm for that backend was inactive and fell through to the wildcard, returning "Unknown cache level: 'X'" instead of a meaningful error.

Found while building Fedora package:

failures:

---- cache::multilevel::test::test_config_level_not_configured stdout ----

thread 'cache::multilevel::test::test_config_level_not_configured' (3797) panicked at src/cache/multilevel_test.rs:621:9:
Expected error about missing config or feature, got: Unknown cache level: 'redis'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    cache::multilevel::test::test_config_level_not_configured

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.33%. Comparing base (c0db872) to head (c78c8a7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2695      +/-   ##
==========================================
- Coverage   74.34%   74.33%   -0.02%     
==========================================
  Files          70       70              
  Lines       39373    39373              
==========================================
- Hits        29271    29266       -5     
- Misses      10102    10107       +5     

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