fix: improve error message for corrupted backend state

[vietcgi]

Summary

• Improves error message when backend state file contains Terraform state data instead of backend configuration

Test plan

• Unit tests added for new error detection
• All existing tests pass
• Real-world scenario tested manually

Fixes #37953

[crw]

Hi @vietcgi, we agree this is an issue, however, at triage the consensus is that we should seek a different solution. Specifically, disallow the local backend from saving state files in the .terraform folder. This would be achieved when validating the local backend’s config, prevents users creating the situation in the first place. This solution would give the team more freedom around evolving backend state files in future Terraform versions.

Please feel free to open a new PR with the above solution! We would be happy to review it. Feel free to comment if you have any questions. Thanks!