Backport of core: Fix double-marked sensitive attributes into v0.15

[teamterraform]

Backport

This PR is auto-generated from #28381 to be assessed for backporting due to the inclusion of the label 0.15-backport.

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

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If a resource has a writeable attribute which is sensitive in the schema, which is bound to a marked sensitive value, reading the resource from state would previously crash due to double-marking the value. To fix this, we need to unmark the value from state, merge its marks with the schema-derived marks, and re-apply the merged marks.

Fixes #28370.

[codecov]

Codecov Report

Merging #28383 (acc5c44) into v0.15 (fe2966a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
terraform/evaluate.go	53.18% <100.00%> (+0.60%)	⬆️
checkpoint.go	2.63% <0.00%> (-28.95%)	⬇️
terraform/node_resource_plan.go	98.05% <0.00%> (+1.94%)	⬆️
internal/providercache/dir.go	73.46% <0.00%> (+6.12%)	⬆️

[github-actions]

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