fix(cli): incorrect cdk diff output for nested stacks

[sai-ray]

Fixes #1228

cdk diff produces incorrect output for nested stacks when the mangled character filter triggers. The mangled diff computation in formatStackDiffHelper uses this.oldTemplate (root stack's deployed template) instead of the oldTemplate parameter (nested stack's deployed template). This causes the correct nested diff to be overwritten with a meaningless comparison between the root and nested templates.

The fix replaces this.oldTemplate with oldTemplate on one line.

Added unit test for nested stack mangled character filtering.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

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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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.94%. Comparing base (ae037f9) to head (1b62697).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1229   +/-   ##
=======================================
  Coverage   87.94%   87.94%           
=======================================
  Files          74       74           
  Lines       10339    10339           
  Branches     1377     1377           
=======================================
  Hits         9093     9093           
  Misses       1220     1220           
  Partials       26       26           

Flag	Coverage Δ
suite.unit	87.94% <ø> (ø)

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