feat: Add pull_request_creation_policy to github_repository

[RoFz]

Resolves #3348

---

Before the change?

• github_repository could not manage pull_request_creation_policy.
• Users who needed to enforce it had to rely on out-of-band API calls or local workarounds.

After the change?

• Add pull_request_creation_policy to github_repository as an optional, computed argument.
• Support the all and collaborators_only values.
• Read and update the setting through GitHub GraphQL so the provider can manage it even though the current REST client model does not expose the field.
• Resolve the GraphQL read using the effective repository owner and skip hard failure when the remote GraphQL schema does not expose pullRequestCreationPolicy.
• Add helper coverage, repository acceptance coverage, and resource documentation for the new argument.

Evidence

• go test ./github -run 'TestPullRequestCreationPolicyMapping|TestRepositoryPullRequestCreationPolicyGraphQL|TestIsUnsupportedPullRequestCreationPolicyError' -count=1
• make test
• GH_TEST_AUTH_MODE=individual GITHUB_OWNER=RoFz GITHUB_USERNAME=RoFz make testacc T='TestAccGithubRepository/check_pull_request_creation_policy'
• make lintcheck
• Manual test with a local provider override on disposable repo tf-pr-creation-policy-manual-20260419-120606: terraform apply succeeded, GitHub API readback returned collaborators_only, and a follow-up terraform plan -detailed-exitcode returned no changes.

Pull request checklist

• Schema migrations have been created if needed
• 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?

• Yes
• No

---

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[deiga]

The github_repository resource relies solely on the REST API currently. If you need to use GQL API, then please do that in a separate resource.