Add test coverage for past expires_on in Managed Identity (#940)

* Add test coverage for past expires_on in Managed Identity

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Clarify subsequent-acquisition comment in past expires_on test

Follow-up to review feedback: the second comment overstated that the token is not cached; the test proves it is not served (endpoint is re-hit), not that no entry is written.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>