ci: install the declared test group in the conda job

[Marius1311]

Stacked on #1371 — GitHub retargets this to main once that merges.

Problem

The PETSc / SLEPc + R job hand-rolls its test dependencies in the workflow:

pip install pytest pytest-cov pytest-mock pytest-timeout coverage igraph leidenalg pillow

Every other job installs the test dependency group from pyproject.toml, so the two drifted in both directions:

• leidenalg is dead weight. It appears exactly once in the repo — that line. _cluster_X pins sc.tl.leiden(..., flavor="igraph") and the plotting tests pass clustering_kwargs={"flavor": "igraph"}, so only igraph (already in the group) is ever needed. It was added in CI workflow modernization #1291 (Feb 2026).
• pytest-timeout is missing from the group, even though it is a real test dependency — the conda job runs pytest --timeout=600, and hatch test -- --timeout=… fails locally because the hatch envs don't have it.

Changes

• Declare pytest-timeout in the test dependency group.
• Install that group in the conda job: pip install -e '.[plot]' --group test (pip on the runner is 26.2.1; --group needs ≥ 25.1).

This matches cookiecutter-scverse, whose test workflow installs declared groups only (hatch env create + hatch run …:run-cov) and never an ad-hoc package list.

Note the conda job now also gets jax, moscot and scvelo from the group, so the tests that currently skip there run under the PETSc/SLEPc + R stack too — more coverage, at the cost of a somewhat longer install.

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