In #31597 we changed `cdk diff` to always use the file asset publishing role, instead of direct CLI credentials. This included a refactor that impacted `cdk import`, which was now not uploading the stack template at all anymore. The operation that is now broken only happens in a case with interactive input, which is why this wasn't caught by integ tests.

In this change, put the requisite asset-handling code around `makeBodyParameter` to make the asset uploading happen properly.

In future PRs:

- Add an integration test for `cdk import` which would have exposed the same error.
- Refactor the contract of `makeBodyParameter`, and perhaps more around asset uploading, to make the expectations and promises of that function more clear; right now it was not obvious what the function would and wouldn't do for you, which led to this error.

I did some refactorings in this PR already (renames, removing an unused argument). I saw an opportunity for more but didn't want to add risk and delay to this patch. Hence, forthcoming 😄 .

Closes #31716.

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