feat: Add github_enterprise_app_installation resource

[slang25]

Resolves #3460

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Before the change?

There is no way to install a GitHub App on an organization with access to all repositories via Terraform. github_app_installation_repositories only manages individual repositories on an existing installation, which is impractical for organizations with many repositories and doesn't model the "all repositories" desired state.

After the change?

A new github_enterprise_app_installation resource manages GitHub App installations on enterprise-owned organizations via the enterprise organization-installations API, available on GitHub Enterprise Cloud and GitHub Enterprise Server 3.19+ (verified against the GHES 3.19/3.20 OpenAPI descriptions). The APIs are already present in go-github v89, so no new dependencies.

resource "github_enterprise_app_installation" "example" {
  enterprise_slug      = "my-enterprise"
  organization         = "my-org"
  client_id            = "Iv1.abc123"
  repository_selection = "all" # or "selected" / "none"
}

Implementation notes:

• Resource ID is <enterprise_slug>:<organization>:<client_id>, so imports need no installation ID lookup; installation_id and app_slug are computed.
• all ↔ selected changes update in place via the toggle endpoint; repository set changes while staying selected use the grant/revoke endpoints (adding before removing so the selection is never empty). Transitions involving none force recreation, since the toggle endpoint only accepts all/selected.
• Repository lists are chunked to respect the API's 50-repositories-per-request limit.
• Plan-time validation requires selected_repositories exactly when repository_selection = "selected".
• Acceptance tests are gated on a new GH_TEST_ENTERPRISE_APP_CLIENT_ID env var (the client ID of an installable app) alongside the existing enterprise test mode, so they skip cleanly where not configured.

I don't have access to an enterprise environment, so the CRUD paths have not been exercised against a live API — I'd appreciate a run of the enterprise acceptance tests. Happy to adjust naming/semantics if maintainers prefer a different shape; I noted #3460 is currently labeled blocked pending enterprise-scope work, but this follows the same enterprise_slug pattern as existing enterprise resources (e.g. github_enterprise_organization).

This PR was developed with AI assistance (Claude); I have reviewed the changes.

Pull request checklist

• Schema migrations have been created if needed (example) — not needed, new resource
• 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

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🤖 Generated with Claude Code

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