fix: reject eager_only with an already-narwhals LazyFrame

[gaoflow]

Closes #3226.

from_native has an early-return for an object that is already a narwhals DataFrame/LazyFrame, returning it before the eager_only / eager_or_interchange_only validation runs (_translate_if_compliant enforces it, but the early return skips that path). So a native pl.LazyFrame was correctly rejected:

nw.from_native(pl.LazyFrame(), eager_only=True)  # TypeError ✅

but an already-narwhals one slipped through and was returned unchanged:

lf = nw.from_native(pl.LazyFrame())
nw.from_native(lf, eager_only=True)  # returned the LazyFrame ❌ (should raise)

This adds the same guard in the early-return path: a narwhals LazyFrame with eager_only/eager_or_interchange_only now raises (or is returned unchanged under pass_through=True), while a narwhals DataFrame (which is eager) is still accepted. Regression test added (fails on main).

One note: narwhals.stable.v1.from_native has the same early-return short-circuit (before it calls _from_native_impl), so the v1 entrypoint still passes a narwhals LazyFrame through. I left v1 untouched since it's the frozen stable API and the fix there needs the pass_through resolution moved above the early return — happy to apply the same guard there if you'd like.

[gaoflow]

Pushed dbdc2f24 to fix the typing failure: the nw.from_native(lf, eager_only=True) call in the runtime negative test is intentionally rejected by the overloads, so it now has the same # type: ignore[call-overload] treatment used elsewhere in this file.

Local verification:

• uv run --extra polars pytest tests/translate/from_native_test.py::test_init_already_narwhals_lazy_eager_only -q -> 1 passed
• uv run --extra polars pytest tests/translate/from_native_test.py -q -> 77 passed, 4 skipped
• make typing -> pyright, mypy, and pyrefly all pass
• uv run ruff check tests/translate/from_native_test.py -> all checks passed

Remote typing (3.12, ubuntu-latest) is now green. The remaining fairlearn failure is happening before Narwhals is installed or tested: the downstream job fails in install-deps with File not found: requirements.txt after cd fairlearn, and both install-narwhals-dev and run-pytest are skipped.