fix(ci): pass codecov token to unit test workflow.

[Hweinstock]

Problem

#1775

Summary is that codecov uploads are passing on PRs, but not on push to refactor branch due to missing token error.
Root cause is that we aren't passing the token through the workflow call confirmed by fix testing below.

Solution

• pass secret to unit test job so that it can upload.
• also only upload a single report, since AFAICT codecov is discarding them for the same commit anyway.

Testing

• pushed to this branch directly, overwrote the trigger to include fix/codecov-secrets, and observed successful codecov publish: https://github.com/aws/agentcore-cli/actions/runs/29606846554/job/87972031386.
• I can also see both events showing up separately in codecov: https://app.codecov.io/github/aws/agentcore-cli/commits.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (refactor@22edb1a). Learn more about missing BASE report.

Additional details and impacted files

@@             Coverage Diff             @@
##             refactor    #1792   +/-   ##
===========================================
  Coverage            ?   93.57%           
===========================================
  Files               ?      115           
  Lines               ?     6681           
  Branches            ?        0           
===========================================
  Hits                ?     6252           
  Misses              ?      429           
  Partials            ?        0           

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