test: mock network calls in test_retrieve

[Copilot]

test_retrieve was hitting real external URLs (irimodel.org, chain-project.net), making the test suite flaky, slow, and broken in restricted CI environments.

Changes

• Mock wget.download via unittest.mock.patch so no network I/O occurs
• Tarball cases (00_iri.tar, 00_ccir-ursi.tar): create a minimal in-memory tar fixture using io.BytesIO + tarfile, exercising the full extract-and-delete path in update.retrieve
• Plain-file cases (apf107.dat, ig_rz.dat): write a local stub file, exercising the non-tar path
• Isolated temp dirs: each parameterized case gets its own TemporaryDirectory instead of sharing one across the class
• Call assertion: verify wget.download receives the correct f"{url}/{filename}" and out arguments

with patch("pyiri2016.api.update.wget.download", return_value=fake_path) as mock_download:
    update.retrieve(url, filename, directory=tmpdir)
    mock_download.assert_called_once_with(
        f"{url}/{filename}", out=tmpdir, bar=ANY
    )

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