chore(test): register the pytest markers the suite already uses

The suite applies markers that were never declared in
[tool.pytest.ini_options] markers. pytest only warns for an unknown marker, so
the warning scrolls past unnoticed and a dropped or misspelt registration is
silent.

Registers the ones in use, with descriptions that say what they do: collection
grouping, not ordering. `feature` labels feature-area tests and
`benchmark_relation` marks relation benchmarks; neither affects scheduling, so
neither substitutes for the xdist_group marker that actually serialises a suite.

usefixtures needs no entry and is not added: pytest provides it, so it was a
false positive in the isolation audit rather than a missing registration.

Author: vistart

pyproject.toml

@@ -91,6 +91,9 @@ markers = [
     "requires_capability: marks tests requiring specific backend capabilities",
     "requires_protocol: marks tests requiring specific backend protocol support",
     "requires_inner_join: marks tests requiring INNER JOIN support",
+    "feature: category label for feature-area tests; changes collection grouping only, not ordering",
+    "benchmark_relation: marks relation benchmarks; changes collection grouping only, not ordering",
+
 ]
 
 [tool.coverage.run]