Add regression test: init.ps1 replaces a pre-loaded GitHub module version with the selected one

[MariusStorhaug]

Context

PR #100 makes GitHub-Script's init.ps1 deterministically load the GitHub module version selected via the Version/Prerelease inputs: it removes any already-loaded instances and imports the resolved version with -RequiredVersion -Force -Global. The core guarantee is that a different GitHub module version already imported before the action runs gets replaced by the selected one.

The existing ActionTest matrix covers version selection (exact / range / already-installed), but there is no regression test for the specific scenario the change fixes: a different GitHub module version is already imported into the session before init.ps1 runs.

Request

Add an integration test that:

• Pre-imports a GitHub module version that differs from the one the action will select (e.g. Import-Module GitHub -RequiredVersion <old> before invoking the action, or via a Prescript/setup step).
• Runs the action with Version pinned to a different version.
• Asserts that after init the loaded module (Get-Module GitHub) is exactly the selected version and that no other version remains loaded.

Acceptance criteria

• A test fails against the pre-🩹 [Patch]: Ensure the selected GitHub module version is the one loaded #100 behavior (unconstrained import / skip-when-loaded) and passes with the fix.
• The test asserts the single loaded version equals the selected version.
• Runs in the existing ActionTest workflow matrix.

Raised from Copilot review feedback on #100.