test: turn unexpected warnings into errors

[schloerke]

The unit suite was emitting 7 warnings. This clears them and adds
filterwarnings = error to pytest.ini so new ones fail the suite instead of
scrolling past.

What was warning

• render.download deprecation (3 tests). Three test_renderer.py
  validation tests still decorated with the deprecated render.download; they
  now use render.download_button. The deprecation itself is still covered by
  test_render_download_is_deprecated.
• Offcanvas accessibility (2 tests). The two anonymous-panel
  show_offcanvas() tests legitimately trigger the "should have an
  aria-label" warning — show_offcanvas() on bare content has no way to set
  a title — so they now assert it with pytest.warns() rather than leak it.
• polars cat.get_categories() (2 tests). Not ours to fix — see below.

The narwhals bug this turned up

Chasing the last warning found a real upstream problem, filed as
narwhals-dev/narwhals#3895.

serialize_dtype() in shiny/render/_data_frame_utils/_tbl_data.py calls
col.cat.get_categories() to send a categorical column's levels to the client.
narwhals' polars cat namespace is a pure passthrough — class PolarsSeriesCatNamespace(PolarsSeriesNamespace, PolarsCatNamespace): ..., an
empty body dispatching via __getattr__ — so that lands verbatim on polars'
own Series.cat.get_categories(), deprecated as of polars 1.44 and documented
for removal in polars 2.0. It isn't only test noise: any app rendering a
polars categorical column prints this DeprecationWarning to its user's
console, and there's nothing the app author can do about it.

I prototyped a fix in _tbl_data.py and then threw it away, because every
version of it was polars-specific, which defeats the point of going through
narwhals at all:

• Branching on col.implementation.is_polars() to read the native series works,
  but puts backend-specific code in a module whose whole job is being
  backend-agnostic.
• col.unique(maintain_order=True) is backend-agnostic but wrong: pandas
  categoricals carry declared categories, including unused ones and in
  declared order, and two existing test cases cover exactly that.
• dtype.categories — what polars' own deprecation message recommends — raises
  AttributeError: 'narwhals.stable.v1._dtypes.Enum' object has no attribute '_cached_categories', because narwhals/stable/v1/_dtypes.py skips
  NwEnum.__init__. We import narwhals.stable.v1, so the documented
  workaround is unavailable to us. That's noted as a secondary item on the
  issue.

So the upstream issue is the fix, with a scoped ignore in pytest.ini holding
the line until it lands. It carries a runnable reprex, its real output, a
suggested implementation for the polars backend (Enum → pl.Series(dtype.categories),
Categorical → unique(maintain_order=True).drop_nulls().cast(pl.String), both
verified to return today's values), and an offer to send the PR.

Note on filterwarnings = error

The tradeoff: a new pandas/polars/starlette release that adds a
DeprecationWarning will turn CI red on an unrelated PR. The fix is one ignore
line, and the red is usually a real signal — as the narwhals case shows, a
warning in our test output was a warning in users' app consoles. Every ignore
carries a comment so the list stays prunable.

CI turned up a second, more awkward class, which the later commits here ignore:

• ResourceWarning from a leaked TemporaryDirectory (failed the oldest-deps
  job, blamed on test_theme_css_compiles_and_is_cached).
• PytestUnraisableExceptionWarning wrapping ValueError: <Token ...> was created in a different Context from an abandoned
  ExtendedTask._done_callback coroutine (failed 3.12/macOS, blamed on
  test_otel_reactive_execution.py).

Both are raised from finalizers at GC time, so they fail whichever test happens
to be running when the collector fires rather than the test at fault — flaky
rather than strict, so they're ignored as a class. Warnings raised on a real
call stack are still fatal, which is where the value is.

The ExtendedTask one looks like a genuine latent bug (a context token reset
from the wrong context during teardown), currently invisible because it only
happens in an abandoned coroutine. Out of scope here; worth its own issue.

No library code changes; test config and tests only.