fix(toolkit-lib): hotswap deployments fail with TypeError when a nested stack output joins a List<> parameter

[9pace]

Fixes #1829

Hotswap deployments crash with an uncaught TypeError: evaluatedArgs.join is not a function when a nested stack output uses Fn::Join on a List<>-typed parameter. The nested-stack evaluator was seeded with parameter declarations (e.g. {"Type": "AWS::SSM::Parameter::Value<List<String>>"}) instead of the values from the parent resource's Properties.Parameters, and the resulting TypeError escaped the CfnEvaluationException-only handler, aborting the deployment instead of falling back to CloudFormation.

Two-part fix:

1. Seed the nested evaluator with Properties.Parameters from the parent resource, so Ref resolves to actual values.
2. Add assertEvaluatesToArray / assertEvaluatesToString guards to Fn::Join, Fn::Split, and Fn::Select, throwing CfnEvaluationException (caught, triggers CloudFormation fallback) instead of a raw TypeError. The guards are required even with the seed fix: a nested stack resource without Properties.Parameters seeds {}, Ref falls through to the declaration's Default (a non-array), and only the guard prevents the crash.

Known limitation: CloudFormation's nested-stack Parameters map is Map<String, String>, so list values arrive as "val1,val2,val3" and fall back to CloudFormation instead of hotswapping (possible follow-up).

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• Unit tests added/updated

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (750d650) to head (9bde153).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1842      +/-   ##
==========================================
- Coverage   90.32%   90.29%   -0.04%     
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  Files          80       80              
  Lines       12124    12124              
  Branches     1716     1714       -2     
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- Hits        10951    10947       -4     
- Misses       1139     1143       +4     
  Partials       34       34              

Flag	Coverage Δ
suite.unit	90.29% <ø> (-0.04%)	⬇️

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