Use a dedicated environment for ruff-lsp tests (#27856)
The legacy `ruff-lsp` compatibility job installs the archived project's
entire development environment, including `mypy` and `typed-ast`, even
though it only runs pytest. Those frozen development dependencies
require source builds on Python 3.12.
Add a locked `ruff-lsp-test` dependency group containing the runtime and
test dependencies. Keep the suite on Python 3.12, the newest version
covered by the archived project's CI, and leave the published `ruff`
package out of the environment so the tests exercise the binary built
from this checkout.
This is the base for astral-sh/ruff#27861, which requires wheels for
Python tooling dependencies.
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Co-authored-by: zaniebot <242828183+zaniebot@users.noreply.github.com>
