chore: release fails because of runtime dependency on private packages

[mrgrain]

Fixes #

aws-cdk's bundled-only packages (@aws-cdk/private-tools, @aws-cdk/cdk-explorer) were declared as runtime dependencies, so they stay pinned at projen's ^0.0.0 placeholder (they're never published). gather-versions in cdklabs-projen-project-types now fails releases loudly on any 0.0.0 runtime/peer dependency (cdklabs/cdklabs-projen-project-types#960), breaking our release: https://github.com/aws/aws-cdk-cli/actions/runs/30441825065/job/90542709045

Moves both packages to devDependencies instead, since they're inlined by the bundler and not needed as real runtime deps.

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.

OpenSSF Scorecard

Package	Version	Score	Details
npm/@aws-cdk/cdk-explorer	^0.0.0	Unknown	Unknown
npm/@aws-cdk/private-tools	^0.0.0	Unknown	Unknown
npm/esbuild	^0.28.1	Unknown	Unknown

Scanned Files

• packages/aws-cdk/package.json

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (957c309) to head (c11b848).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1776   +/-   ##
=======================================
  Coverage   90.05%   90.06%           
=======================================
  Files          78       80    +2     
  Lines       12110    12117    +7     
  Branches     1698     1699    +1     
=======================================
+ Hits        10906    10913    +7     
  Misses       1175     1175           
  Partials       29       29           

Flag	Coverage Δ
suite.unit	90.06% <100.00%> (+<0.01%)	⬆️

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