test: force headless Matplotlib backend for local + CI test runs

[petercorke]

Problem

Running pytest locally pops up real Matplotlib windows, and a couple of
animation tests hang outright. CI already sets MPLBACKEND=Agg in the
workflow env, but that never applied to local runs — several modules
(geom2d, geom3d, spline, animate) import matplotlib.pyplot at
module load time, so whichever backend is active at that point sticks for
the whole session, and locally that's the platform's interactive backend.

Fix

1. tests/conftest.py (new): os.environ.setdefault("MPLBACKEND", "Agg")
   before anything else runs. conftest.py is guaranteed to load before
   pytest imports any test module, so this closes the gap for local runs
   without touching the CI workflow (redundant there now, not conflicting).
   setdefault(), not a hard override, so MPLBACKEND=MacOSX pytest ...
   still shows a real plot when you actually want to eyeball one.

2. Forcing Agg everywhere surfaced a real, previously-masked hang:
   tranimate(..., wait=True) busy-loops on plt.pause() waiting for
   FuncAnimation's timer callback to deregister itself — driven by the
   GUI event loop, which Agg doesn't have. Confirmed with a hard-alarm
   timeout: the affected test ate a full 12s budget with zero output vs.
   completing normally under a real backend. These tests were already
   skipped in CI via os.environ.get("CI") == "true", which happened to
   also mask this locally-only-when-Agg-is-forced hang — but only in CI;
   locally (pre-Agg) they ran fine because a real backend genuinely pumps
   the event loop.

   Broadened the three affected skip conditions (test_animate in
   test_transforms3d_plot.py; the two animate=True test_visualize
   tests and the inline guard in test_spline.py) to additionally skip
   when plt.get_backend() == "agg" — the actual root cause, rather than
   the CI-env-var proxy for it. Purely additive, no existing condition
   removed, so CI behaviour is unchanged. Everything else (plain,
   non-animated plotting tests) now safely runs headless both locally and
   in CI — strictly more coverage than before.

Testing

• Full suite, CI unset (simulating a local run): 339 passed, 3 skipped,
  1.6s, no windows, no hangs.
• Full suite, CI=true (simulating CI): 338 passed, 4 skipped — matches
  prior CI behaviour.
• black --check clean at the pinned 23.10.0.

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