Use provided axios instance for credentials refresh operations

[Siddhant-K-code]

Description

This PR ensures that when users provide a custom axios instance to OpenFgaClient, it's consistently used throughout all SDK operations - including credential refresh requests. Previously, credential refresh operations were using the global axios instance, which caused issues for users with custom network configurations like VPC or proxy setups.

References

fixes #183

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

[wassil]

Thank you for doing this!
Looks good to me, I did it the same way when fixing this locally.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.65%. Comparing base (006b5c4) to head (691afea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #193   +/-   ##
=======================================
  Coverage   87.65%   87.65%           
=======================================
  Files          23       23           
  Lines        1142     1142           
  Branches      198      179   -19     
=======================================
  Hits         1001     1001           
  Misses         83       83           
  Partials       58       58           

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[ewanharris]

Thanks @Siddhant-K-code!