test(e2e): fix test and cover new sign method

[jeanregisser]

Follow up from: #98

Since secrets aren't shared with external PRs, I manually tested locally but missed running the e2e test itself.
And then it failed once merged into main: https://github.com/valora-inc/viem-account-hsm-gcp/actions/runs/13139225860/job/36661999051

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.65%. Comparing base (bdf263a) to head (3b4840d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   93.33%   93.65%   +0.31%     
==========================================
  Files           2        2              
  Lines          60       63       +3     
  Branches        6        6              
==========================================
+ Hits           56       59       +3     
  Misses          4        4              

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[github-actions]

🎉 This PR is included in version 1.2.0 🎉

The release is available on:

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• GitHub release

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