test(aws-cdk): mock agent detection instead of clearing its env vars

[9pace]

The auto-default tests in cli.test.ts cleared a hardcoded copy of the env vars guessAgent() reads. The copy went stale when #1855 added AI_AGENT/AGENT, making the "no agent detected" test fail when the suite runs inside an agent that sets them.

Mock guessAgent() instead: these tests verify the guard in exec(), and detection has its own coverage in guess-agent.test.ts. New detection signals no longer require touching this file.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (a73eebf) to head (96938b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1966      +/-   ##
==========================================
- Coverage   91.32%   91.29%   -0.04%     
==========================================
  Files          79       79              
  Lines       12217    12217              
  Branches     1724     1722       -2     
==========================================
- Hits        11157    11153       -4     
- Misses       1025     1029       +4     
  Partials       35       35              

Flag	Coverage Δ
suite.unit	91.29% <ø> (-0.04%)	⬇️

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