test: isolate deprecation unit test from live NASA URS calls

[mvanhorn]

Description

tests/unit/test_deprecations.py made real network calls to NASA's URS login endpoint. Its auth fixture mocked the URS calls only while constructing the auth object, but the responses context exited before the test body ran, so earthaccess.login(...) and Store(auth) inside the test reached the live service. That makes a "unit" test depend on network access and valid credentials, so it fails or hangs in offline/CI sandboxes.

This keeps the mocked URS responses active for the whole test by making the auth fixture yield from inside a function-scoped responses.RequestsMock context. Every request on the login/Store path is now served by the mock, and any unmocked request is blocked rather than sent to NASA. The test's assertions are unchanged; only its network isolation is fixed. No production code changes.

resolves #1321

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Merge checklist

• PR title is descriptive
• PR body contains links to related and resolved issues (e.g. closes #1)
• If needed, CHANGELOG.md updated (not needed — test-only change, no user-facing behavior change)
• If needed, docs and/or README.md updated (not needed)
• If needed, unit tests added (unsure how? see below!) (this PR repairs the existing unit test's network isolation)
• All checks passing (tip: comment pre-commit.ci autofix if pre-commit is failing) (pytest tests/unit/test_deprecations.py passes locally with no network)
• At least one approval

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📚 Documentation preview 📚: https://earthaccess--1370.org.readthedocs.build/en/1370/

[github-actions]

👈 Launch a binder notebook on this branch for commit f248c0b

I will automatically update this comment whenever this PR is modified

[Sherwin-14]

Thanks @mvanhorn! This looks so much cleaner now.