helper/resource: include step number in import state verification failures

[massimomazzariol]

Related Issue

Fixes #525

Description

When import state verification cannot find the imported resource ID in the prior state, the verification currently calls t.Fatalf directly. This prevents the outer test step runner from adding the step number to the failure message.

This change returns the verification error instead, allowing the existing import step error handling to report the failure with its step number.

A regression test was added to verify that the final error includes Step 2/2, along with a Changie entry for the bug fix.

Testing

The import state test package passes with both Go versions used by the repository CI matrix:

• GOTOOLCHAIN=go1.25.12 go test ./helper/resource/importstate -count=1
• GOTOOLCHAIN=go1.26.5 go test ./helper/resource/importstate -count=1

Tested with:

• Windows 11 Pro 10.0.26200
• Terraform 1.15.8
• Go 1.25.12 and 1.26.5
• Git 2.54.0.windows.1

go test ./... -count=1 also encounters pre-existing Windows-specific failures in config, helper/resource, and internal/teststep. The same failures were reproduced on an unmodified upstream/main worktree.

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

No changes to security controls.

[hashicorp-cla-app]

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