docs: bucket deletion cascades — update limitation comments

[sorenbs]

Linked issue

n/a — small change

Summary

The Management API is gaining cascade semantics for bucket deletion (a bucket is deleted together with its objects, matching database deletion) via an in-flight pdp-control-plane change. This updates the two comments that documented the old "non-empty bucket delete returns 500" limitation: the Bucket provider's delete hook now documents the cascade contract, and the e2e verify's cleanup DELETE is re-justified as CRUD-round-trip coverage rather than a destroy prerequisite.

Merge after the control-plane cascade change deploys — until then the old limitation still holds in production.

Testing performed

• Comment-only diff; pnpm turbo typecheck --filter @internal/lowering clean locally.
• The bucket e2e job on this PR exercises the same deploy/verify/destroy path either way (verify empties the bucket before destroy, so it passes under both old and new server semantics).

Checklist

• All commits are signed off (git commit -s) per the DCO. The DCO status check will block merge if any commit is missing a Signed-off-by: trailer.
• I read CONTRIBUTING.md and the change is scoped to one logical concern.
• The PR title is a conventional commit (feat, fix, chore, docs, refactor, test, build) — PR titles drive the auto-generated release notes.
• Tests are updated (or n/a if the change is doc-only / refactor with no behavioural delta). — n/a, comment-only.

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