docs: state the equality as a rule, not as a defence of it

The paragraph explained why the assertion is not a subset check, which only
reads as an explanation to someone holding the previous version. Say what a
maintainer should do instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/tests/test_context.py

@@ -1681,12 +1681,9 @@ def test_every_component_field_has_an_installer():
     ``_collect_contributions``, and resolve and install it in
     ``with_extensions``. Then add it below.
 
-    The dataclass is the source of truth, so the comparison is an equality and
-    not a subset — a subset would grow to cover a new field on its own, which
-    is the one case this exists to catch. A field that is deliberately not a
-    component collection goes in ``not_components`` rather than into
-    ``_COMPONENT_NOUNS``, which is what keeps that exemption a decision someone
-    made instead of an operator someone loosened.
+    Keep the comparison an equality. A subset check is satisfied by any new
+    field on its own, which is the one case this exists to catch. A field that
+    is deliberately not a component collection belongs in ``not_components``.
     """
     from datafusion.extensions import _COMPONENT_NOUNS