fix: improve check for validity of token

[ewanharris]

Description

• 1d20960 - Extends the testing of the token validity check to include checks for closer to the expiry time (expires in 6 minutes, 5 minutes)
• 2084b0a - Refactors the validity check to split it out into separate null checks and time checks.

I've removed the jitter here as I'm not sure what it added and it made testing harder as we didn't have predictability. If we want to guard against a potential "thundering herd" on the token refresh then I think we'd be better off attempting to only allow one call to the token endpoints to occur at a time (not sure how to do this in Java myself, maybe if we used synchronized on exchangeToken?

The jitter calculation has been changed to use nextInt(TOKEN_EXPIRY_BUFFER_THRESHOLD_IN_SEC), this will return a value up to our 300 seconds limit that we can use to prevent multiple calls at the same time. From my testing nextLong could potentially return negative values so I believe this is not safe to use, given we want our jitter to be up to 300 seconds more then we nextInt with the bound is safer.

References

Resolves #75

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.65%. Comparing base (6717967) to head (f98569c).

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #76      +/-   ##
============================================
+ Coverage     28.51%   28.65%   +0.13%     
- Complexity      721      723       +2     
============================================
  Files           145      145              
  Lines          5277     5283       +6     
  Branches        567      567              
============================================
+ Hits           1505     1514       +9     
+ Misses         3714     3712       -2     
+ Partials         58       57       -1     

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