feat(cli): support --change-set-name for cdk diff

[mrgrain]

cdk deploy, cdk watch, and cdk import all accept --change-set-name, but cdk diff always creates its change set with the hardcoded name cdk-diff-change-set. In automated environments where multiple diffs can run against the same stack, or where change set names are used for auditing, callers need to control that name — same motivation that added the flag to deploy in the first place.

This adds a changeSetName option to toolkit-lib's ChangeSetDiffOptions and plumbs it through to createDiffChangeSet, keeping cdk-diff-change-set as the default so existing behavior is unchanged. The CLI exposes it as --change-set-name on cdk diff, and rejects the combination with --method=template (where no change set is created), consistent with deploy rejecting it for --method=direct. createValidationChangeSet explicitly Omits the new option, since the validation change set used by cdk diagnose derives its name from a per-run UUID.

Verified with new unit tests in both packages (custom name reaches CreateChangeSetCommand; CLI passes the flag through and rejects incompatible methods) plus full green builds including the toolkit-lib API Extractor check.

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:

---

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 91.31%. Comparing base (8904fc5) to head (6b117a4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1982      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.02%     
==========================================
  Files          79       79              
  Lines       12215    12233      +18     
  Branches     1724     1725       +1     
==========================================
+ Hits        11155    11170      +15     
- Misses       1025     1027       +2     
- Partials       35       36       +1     

Flag	Coverage Δ
suite.unit	91.31% <100.00%> (-0.02%)	⬇️

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[rix0rrr]

Shouldn't we also switch to cdk-diff-change-set-${uuid()} by default, to solve the 99% case automatically?

[rix0rrr]

Shouldn't we also switch to cdk-diff-change-set-${uuid()} by default, to solve the 99% case automatically?