test(807): make "threaded through every branch" a checked property

`_resolve` returns three sets and the third has to reach every composition site.
That was a claim in a docstring; each site is a separate opportunity to forget,
and forgetting is silent — no error, just a smaller population.

Ten sites enumerated and asserted: top level, `allOf` arm, `if`, `then`, `else`,
`dependentSchemas`, `oneOf` arm, `anyOf` arm, `items`, `prefixItems`. All ten
currently thread it. Witnessed load-bearing by dropping the threading at three of
them — `allOf` reddens 1 case, `oneOf`/`anyOf` 2, `items` 2.

The value is not today's ten passes; it is that a NEW composition keyword has one
visible place to be added to, instead of the omission being spread across the
resolver where nothing would report it.

956 passed 2 skipped, ruff check 0, ruff format 0.

scripts/tests/test_optional_request_members.py

@@ -665,6 +665,43 @@ def test_composition_is_resolved_the_way_JSON_Schema_means_it(case: str, schema:
     assert sorted(_optional_of(schema)) == expected, case
 
 
+_NESTED_PROBE = {
+    "properties": {"outer": {"properties": {"req": {}, "opt": {}}, "required": ["req"]}},
+    "required": ["outer"],
+}
+
+
+@pytest.mark.parametrize(
+    ("site", "schema"),
+    [
+        ("top level", _NESTED_PROBE),
+        ("allOf arm", {"allOf": [_NESTED_PROBE]}),
+        ("if", {"if": _NESTED_PROBE}),
+        ("then", {"then": _NESTED_PROBE}),
+        ("else", {"else": _NESTED_PROBE}),
+        ("dependentSchemas", {"dependentSchemas": {"trigger": _NESTED_PROBE}}),
+        ("oneOf arm", {"oneOf": [_NESTED_PROBE]}),
+        ("anyOf arm", {"anyOf": [_NESTED_PROBE]}),
+        ("items", {"type": "array", "items": _NESTED_PROBE}),
+        ("prefixItems", {"type": "array", "prefixItems": [_NESTED_PROBE]}),
+    ],
+)
+def test_a_nested_inline_object_survives_EVERY_composition_site(site: str, schema: dict) -> None:
+    """`_resolve` returns three sets, and the third has to be threaded through every branch.
+
+    A composition site that unions `properties` and `required` but forgets `nested_optional` loses
+    the nested member silently — no error, just a smaller population — and each site is a separate
+    opportunity to forget. Enumerating the sites here is what makes "threaded through every branch"
+    a checked property instead of a claim in a docstring; a NEW composition keyword must be added
+    to this list, and if it is not, the omission is at least visible in one place rather than
+    spread across the resolver.
+    """
+    assert "outer.opt" in _optional_of(schema), (
+        f"a nested inline object under `{site}` lost its optional member — `nested_optional` is "
+        "not threaded through that branch of `_resolve`"
+    )
+
+
 def test_a_ref_ARM_is_covered_by_the_component_walk_not_by_this_one() -> None:
     """The two walks COMPOSE; neither alone covers an inline body with a `$ref` arm.