feat(integ-runner): detect bootstrap errors and retry tests in valid regions

[mrgrain]

When running integration tests with --parallel-regions, tests fail with unhelpful errors if a region is not bootstrapped. The integ-runner continues scheduling tests for that region, wasting CI time and creating noise in the output.

This PR introduces automatic detection and handling of bootstrap errors:

toolkit-lib changes:

• Add a new BootstrapError class that captures account and region information when bootstrap-related failures occur
• Update existing bootstrap error sites to throw BootstrapError instead of generic ToolkitError

integ-runner changes:

• Detect BootstrapError during test execution using the new detectBootstrapError() function
• Maintain an EnvironmentPool that tracks available vs removed environments
• When a bootstrap error occurs, remove that environment from the pool and re-queue the failed test
• If other valid environments remain, the test is retried there automatically
• If no valid environments remain, the test fails with a clear message
• At the end of the run, print a summary of removed environments with the exact cdk bootstrap command needed

This improves the developer experience by providing actionable feedback and avoiding repeated failures in non-bootstrapped regions.

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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 87.73%. Comparing base (3e25a4e) to head (86fc297).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1115      +/-   ##
==========================================
+ Coverage   87.71%   87.73%   +0.01%     
==========================================
  Files          72       72              
  Lines       10116    10116              
  Branches     1336     1337       +1     
==========================================
+ Hits         8873     8875       +2     
+ Misses       1217     1215       -2     
  Partials       26       26              

Flag	Coverage Δ
suite.unit	87.73% <ø> (+0.01%)	⬆️

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