fix(aws-cdk): warn when legacy exports are used

[mrgrain]

This PR adds a prominent deprecation warning for legacy exports from the aws-cdk package. These exports were never officially supported and are scheduled for removal after 2026-03-01.

Changes

• Added a warning message that displays when legacy exports are imported
• The warning directs users to migrate to the official CDK Toolkit Library
• Includes an environment variable CDK_DISABLE_LEGACY_EXPORT_WARNING=1 to disable the warning if needed
• References issue ⚠️ NOTICE: Deprecation and removal of legacy exports from AWS CDK CLI package #310 for more information

The warning uses ANSI color codes to create a visually prominent yellow banner that clearly communicates the deprecation timeline and migration path.

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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 83.01%. Comparing base (742ccfc) to head (e43eda9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #931      +/-   ##
==========================================
- Coverage   83.44%   83.01%   -0.44%     
==========================================
  Files          71       71              
  Lines       10401    10401              
  Branches     1314     1302      -12     
==========================================
- Hits         8679     8634      -45     
- Misses       1684     1729      +45     
  Partials       38       38              

Flag	Coverage Δ
suite.unit	83.01% <ø> (-0.44%)	⬇️

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