test(lib): ensure TerraformAsset stage within stack own directory

[eduardomourar]

Related issue

Follow-up from #380.

Description

#380 claimed asset staging lands as a sibling of stacks/, breaking once uploaded to a hosted runner. Traced it: TerraformAsset already stages inside stacks/<stack-id>/, and has since before #371. The bug only ever existed in the old, never-merged spike's AssetStaging, which #433 didn't carry forward.

No code fix needed. Adds a regression test pinning this contract so it can't regress silently later (e.g. in #339).

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[jsteinich]

Cheap and worth having. One framing note below, plus: this touches canonical-asset-hash.test.ts, as does #433, so the two will likely conflict — worth picking a merge order.

[eduardomourar]

I would like to merge this PR first as it is smaller.