fix: org detection failure on ghec-dr endpoints

[ebk45]

Resolves #2626

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Before the change?

• When base_url is a GHEC-DR endpoint (https://api.*.ghe.com/), Organizations.Get() fails due to incorrect URL construction in the go-github library. The error is silently discarded, leaving IsOrganization as false. Any resource or data source that calls checkOrganization() (e.g. github_external_groups) then fails with the misleading error: this resource can only be used in the context of an organization, "XYZ" is a user.

After the change?

• When Organizations.Get() fails, the provider logs a [WARN] and falls back to Users.Get() to check the owner's type field. Since /users/{owner} works correctly on GHEC-DR endpoints, the organization is detected and IsOrganization is set to true. Downstream resources work as expected.

Pull request checklist

• [N/A] Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• [N/A] 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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[github-actions]

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