feat(toolkit-lib): register message codes for dependency-stack selection lines

[sai-ray]

This PR adds message codes to two stack-selection lines:

• CDK_TOOLKIT_I1002 — "Including dependency stacks" (upstream)
• CDK_TOOLKIT_I1002 — "Including depending stacks" (downstream)

They were emitted without a code, so integrators couldn't target them. Text is unchanged.

#1663 will use these codes to suppress the lines for cdk list.

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

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[sai-ray]

This is great! Minor note: the CDK_ASSEMBLY_ messages are reserved for messaged originating from the user's CDK app. This message is from the toolkit, despite being from a class called StackAssembly 🙈

Wasn't totally sure where these belonged, so I followed CDK_TOOLKIT_I1001 and put them in the synth band next to it since it's also a stack-selection message that fires from lots of commands and not just synth.