fix: prevent inconsistent final plan for prevent_self_review in github_repository_environment

[kishaningithub]

Fixes an issue where Terraform reports "Provider produced inconsistent final plan" for the prevent_self_review attribute on github_repository_environment resources that have no reviewers configured.

Resolves a missed edge case in #1967

Problem

When an environment has no reviewers, the GitHub API does not return a required_reviewers protection rule. The read function only set prevent_self_review inside the case "required_reviewers" branch, so the attribute was never written to state—leaving it as null. Terraform's plan expected false (the schema default), causing the following mismatch:

Error: Provider produced inconsistent final plan .prevent_self_review: was cty.False, but now null.

Root Cause

1. prevent_self_review was only set in state when a required_reviewers protection rule existed in the API response.
2. pr.PreventSelfReview (a *bool) was passed directly to d.Set() without nil-checking, which could also store null in state.

Fix

• Set prevent_self_review to false before iterating protection rules, ensuring it always has a value in state.
• Safely dereference pr.PreventSelfReview with a nil guard, defaulting to false.

Testing

Environments without reviewers (the failing case) are already exercised by the create_with_id_separator_in_name and update_to_add_reviewers acceptance tests which create environments with no initial reviewers.

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[deiga]

Nice, thanks! Could you add a regression acceptance test case as well?

[Copilot]

Pull request overview

These provider review instructions are being used.

Fixes prevent_self_review state normalization when GitHub omits reviewer protection data.

Changes:

• Defaults prevent_self_review state to false.
• Safely handles a nil API value.

[kishaningithub]

@deiga Have added the test. Can you kindly review?