feat(eap): Default unspecified sampling mode to high accuracy

[narsaynorath]

Exposes HIGHEST_ACCURACY mode and uses it as the default if no sampling mode is defined.

Eventually we want to lean into the fact that NORMAL mode is the default, but the frontend needs to be updated to make use of this. At the moment the frontend passes nothing for a high accuracy request, so we'll enforce the behaviour we expect here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

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##           master   #90704       +/-   ##
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+ Coverage   52.55%   87.84%   +35.28%     
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  Files       10275    10283        +8     
  Lines      583109   583622      +513     
  Branches    22569    22568        -1     
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+ Hits       306478   512694   +206216     
+ Misses     276202    70495   -205707     
- Partials      429      433        +4