Core: Add hint for resource conflict errors (EntityAlreadyExists)

[KyungHwanKim-devs]

Refs #24704

Summary

• Keep the existing diagnostic hint for resource conflict errors, but make it more precise and less noisy.
• Tighten the match to only already exists/EntityAlreadyExists so unrelated duplicate errors do not get a misleading hint.
• Expand the hint message to cover both common root causes: missing moved blocks during refactors and provider-side asynchronous deletes/eventual consistency.
• Update tests and the changelog entry to match the refined behavior and wording.

Rationale / Background

This issue has two recurring causes:

1. A refactor (rename or module move) where Terraform sees the old and new addresses as different resources. Without moved blocks, Terraform plans a destroy+create and may attempt the create before the old object is fully gone.
2. Provider-side delete operations that return before the object is fully removed (eventual consistency), which can surface as already exists errors on replacement.

The hint should steer users toward the correct fix without over-triggering on unrelated errors. This change keeps the hint but narrows the match to explicit already exists signals and clarifies the two likely root causes in the message.

User-facing Behavior

No behavior change to planning or apply execution. A warning hint is appended only when an error explicitly indicates the object already exists.

Testing

• go test ./internal/terraform -run TestHintIfAlreadyExists

Target Release

1.15.x

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. This only adjusts diagnostic messaging.

CHANGELOG entry

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

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[KyungHwanKim-devs]

Hi @radeksimko , CI checks are passing and CLA is signed. Could someone from the team review this when you have a chance? Happy to address any feedback. Thanks!

[crw]

Hi @KyungHwanKim-devs, this PR looks OK to review, the one comment from triage was:

The only thing that I’d want is another core maintainer's opinion on is whether we want to always prioritize the current non-configDirXDG path if both are present. The PR favors the XDG path over the current implementation.

I will raise this again in triage to see if we can prioritize reviewing this submission. Thanks!

[crw]

Hi @KyungHwanKim-devs, the feedback from triage is that this error covers one possibility that could cause this error, but may be misleading to users for other ways in which this error could be triggered. So this could be helpful for the right user, but send other users with this issue down the wrong track. For that reason, we are going to decline accepting this PR. Feel free to follow up if you believe there is a better way to target this hint.