Backport of terraform test: remove marks before passing variables as inputs to a plan into v1.6

[teamterraform]

Backport

This PR is auto-generated from #34190 to be assessed for backporting due to the inclusion of the label 1.6-backport.

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The below text is copied from the body of the original PR.

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This PR removes the sensitive marks from variables before passing them as input variables into run blocks. This only affects situations where users are passing outputs from previous run blocks that are marked as sensitive.

In addition, we add a warning if the receiving variable configuration isn't also marked as sensitive, as the sensitive metadata is being lost if the config isn't also marked as sensitive. It might be we want to actually completely disallow this, so add this diagnostic as an error instead of a warning. I'm happy to make that change if we think it's necessary.

Fixes #34185

Target Release

1.6.4

Draft CHANGELOG entry

BUG FIXES

• terraform test: Fix bug preventing passing sensitive output values from previous run blocks as inputs to future run blocks.

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Overview of commits

• ea5006a - b1c76db - 22302cd

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