Summary

Until now, deciding which tests run with/without hf_xet relied on two mechanisms that were rarely in sync: name-based CI filtering (pytest -k 'test_xet or test_buckets or test_copy_files') and @requires("hf_xet") decorators. In the end it was hard to know if a test ran at all, and local runs (hf_xet installed) differed from CI. This PR replaces both with explicit markers enforced by a single autouse fixture (xet_mode in tests/conftest.py):

• @pytest.mark.xet → test requires hf_xet: skipped if not installed, Xet force-enabled otherwise. Whole modules are marked via pytestmark = pytest.mark.xet (xet upload/download/utils, buckets, copy_files).
• @pytest.mark.no_xet → test must run without hf_xet: skipped if hf_xet is installed. No reliance on HF_HUB_DISABLE_XET — the test only runs in environments where hf_xet is genuinely absent.
• unmarked → test must work regardless of Xet: nothing forced.

CI mapping (ubuntu + windows):

• "Xet only" job (hf_xet installed): -m 'not no_xet' → xet tests + all unmarked tests, with Xet.
• all other jobs (hf_xet uninstalled): -m 'not xet' → no_xet tests + all unmarked tests, without Xet.

So unmarked tests now run twice in CI (once per mode) — that's what actually verifies "works regardless of Xet" instead of just documenting it. Inference tests stay excluded from the Xet job (they have their own job and don't depend on Xet). Marking a test with both markers is a hard failure, and --strict-markers is enabled so a typo'd marker is a collection error.

$ pytest tests -m xet           # 269 tests  (xet-required)
$ pytest tests -m "not xet"     # 2192 tests (what CI runs without hf_xet)
$ pytest tests -m "not no_xet"  # 2461 tests (what CI runs with hf_xet)

Decisions taken

• Unmarked tests run in both CI jobs (rather than only the non-xet job, or only on one matrix entry). Costs more runner minutes on the "Xet only" job (~270 → ~2400 tests) but wall-clock is similar (parallel jobs) and it's the only option where a hidden Xet-dependence in an unmarked test surfaces in CI.
• no_xet tests are skipped when hf_xet is installed, rather than force-disabling Xet via HF_HUB_DISABLE_XET. Symmetric with xet (each marker pins the test to one environment) and doesn't depend on the env-var escape hatch behaving correctly everywhere. Locally (hf_xet installed) no_xet tests skip; CI covers them in the non-xet jobs.
• no_xet currently has zero uses: I couldn't statically identify tests that break with Xet enabled. The first "Xet only" runs are the discovery mechanism — each failure there is either a missing no_xet mark or a real Xet-path bug. Expect some triage on this PR's CI.
• The hf_xet uninstall step in non-Xet jobs is kept: it's what guarantees unmarked tests run Xet-free there, and catches accidental hard imports of hf_xet.

Bugs the old -k filtering was hiding

• TestSkillsMarketplaceCLI (tests/test_cli.py) was @requires("hf_xet") but matched no -k pattern → it never ran in any CI job (only collected where hf_xet was uninstalled → always skipped). Now marked xet and running.
• test_xet_callback_sums_increments_across_concurrent_commits (tests/test_upload_pipeline.py, fully mocked, no hf_xet needed) was routed to the Xet job purely by its name.
• The Windows Xet job only ran -k "test_xet" → buckets/copy_files tests never ran on Windows at all.

Sanity check of the fixture (with hf_xet installed locally):

def test_unmarked_uses_environment():
    assert is_xet_available()       # passed

@pytest.mark.no_xet
def test_no_xet_skipped_when_installed():
    raise AssertionError(...)       # SKIPPED: "Test must run without `hf_xet` installed"

@pytest.mark.xet
def test_xet_forced_on():
    assert is_xet_available()       # passed

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