Update calls to StateMgr method to use diagnostics more conventionally

[SarahFrench]

Follow up to #37589. In that case, the interface defining the StateMgr method was updated to return diagnostics instead of errors, and something went wrong when that change was pulled into a long-lived feature branch.

This PR changes that code and other examples of calling code to handle diagnostics more conventionally:

• Instead of checking for non-nil values, inspect for errors
• Retrieve an error representation of the diagnostics where calling code previously used errors to make test failure messages or return errors to users.

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[SarahFrench]

s3 tests:

$ go test -v github.com/hashicorp/terraform/internal/backend/remote-state/s3

 ...omitted test output...

--- PASS: TestValidateDuration (0.00s)
    --- PASS: TestValidateDuration/valid (0.00s)
    --- PASS: TestValidateDuration/fails_validator (0.00s)
    --- PASS: TestValidateDuration/invalid (0.00s)
--- PASS: TestValidateSetStringElements (0.00s)
    --- PASS: TestValidateSetStringElements/valid (0.00s)
    --- PASS: TestValidateSetStringElements/fails_validator (0.00s)
PASS
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 306.447s

[SarahFrench]

I can't run the kubernetes tests using the usual method, as I think ToB has removed docker from my laptop.

gcs tests:

$ go test -v github.com/hashicorp/terraform/internal/backend/remote-state/gcs

...removed output here...

--- PASS: TestAccBackendWithCustomerSuppliedEncryption (19.44s)
=== NAME  TestAccBackend
    testing.go:303: TestBackend: testing state locking for *gcs.Backend
2025/09/12 17:35:29 Object deleted: bar.tfstate
2025/09/12 17:35:30 Object deleted: default.tfstate
--- PASS: TestAccBackendWithCustomerManagedKMSEncryption (22.29s)
2025/09/12 17:35:34 Object deleted: bar.tfstate
2025/09/12 17:35:35 Object deleted: default.tfstate
--- PASS: TestAccBackend (27.45s)
PASS
ok      github.com/hashicorp/terraform/internal/backend/remote-state/gcs        86.411s

[mildwonkey]

lgtm!

[SarahFrench]

Thanks!

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