fix(flags): use feature_enabled field of unleash variants

[aliu39]

Fixes a logic error in #3888

After rereading https://docs.getunleash.io/reference/strategy-variants and testing w/ a local Unleash instance, I found we need to use a different variant field, feature_enabled. enabled refers to whether the variant itself is enabled, but a feature/"toggle" can be enabled with no variants. In this case, a disabled variant is returned (see no_variant_payload in tests).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.20%. Comparing base (fa241c3) to head (adb3bb1).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3914   +/-   ##
=======================================
  Coverage   80.20%   80.20%           
=======================================
  Files         140      140           
  Lines       15412    15412           
  Branches     2593     2593           
=======================================
  Hits        12361    12361           
- Misses       2206     2207    +1     
+ Partials      845      844    -1     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/unleash.py	93.75% <100.00%> (ø)

... and 1 file with indirect coverage changes

[cmanallen]

@aliu39 How are people using get_variant in practice? Does it make sense to send a true, false value to Sentry if what the user is actually working with strings a, b, or c?