Definition fixture scope and a generic ItemDefinition node (#3926)

[RonnyPfannschmidt]

Note

Stacked on #14769 — that PR's commit is included here. Review only the second
commit, or diff against RonnyPfannschmidt:feature/function-definition-collection-node.

Builds on the opt-in FunctionDefinition collection node in two directions: a
fixture scope that targets that node, and a collector-agnostic base so
non-Python test definitions can be parametrized through the same hook.

1. Scope.Definition

Sits between Function and Class, anchored on the definition node, so one
fixture instance is shared by all invocations generated from a single
definition:

@pytest.fixture(scope="definition")
def resource(): ...

@pytest.mark.parametrize("n", [1, 2, 3])
def test_it(resource, n):
    ...  # one `resource` for all three parameter sets

SETUP    D resource
  test_x.py::test_it[1] .
  test_x.py::test_it[2] .
  test_x.py::test_it[3] .
TEARDOWN D resource

The scope needs the definition to actually be in the tree, i.e.
collect_function_definition = pedantic. Without it, both a definition-scoped
fixture and parametrize(scope="definition") fail with an explicit
ScopeUnavailable message rather than silently degrading to function scope —
there is nothing to fall back to that preserves the meaning of the scope.

--setup-show spells it D. The scope-to-indentation mapping is now written
out explicitly so that adding a scope does not shift the output of all the
existing ones.

2. nodes.ItemDefinition

The collector-agnostic counterpart of FunctionDefinition: a collector standing
for one test definition, whose collect() runs the generate-tests protocol and
turns each callspec into an item.

class YamlDefinition(nodes.ItemDefinition):
    parametrize_argnames = ("value",)

    def make_item(self, parent, *, name, callspec, nodeid, context):
        return YamlItem.from_parent(
            parent, name=name, nodeid=nodeid, spec=dict(callspec.params)
        )

Any ordinary pytest_generate_tests implementation now applies to it, and the
chosen values arrive on item.callspec.params. Ids, pytest.param(...) marks
and node-id selection work as they do for Python tests.

Attaching the callspec, its marks and its id to the generated item is done by
the protocol rather than left to each make_item() — the callspec drives
reordering and high-scoped param caching, so it is not optional.

Not included: fixture resolution for non-Python items — no indirect=True,
no scope inference, no requesting fixtures. doc/en/proposals/generic_fixture_closures.rst
writes up the four concrete couplings that stand in the way (TopRequest
assuming a Python item, signature-driven getfixtureinfo(),
DirectParamFixtureDef desugaring, Function.setup()) and a three-step path.

3. Layering

The parametrization core moves out of python.py into a new
_pytest.parametrize: CallSpec, IdMaker and the new ParametrizeContext.
Metafunc is now the Python-specific subclass of the latter, overriding four
seams (_infer_scope, _validate_argnames, _register_direct_params,
_introspection_target) to bring in the fixture closure, signature validation
and direct-parameter desugaring. nodes and fixtures both import the new
module; no import cycles. _pytest.python.CallSpec2 keeps warning and resolving
as before.

Two bugs found while testing, fixed here

• Node-id selection is broken by Opt-in FunctionDefinition collection tree node (#3926) #14769 as it stands. pytest test_x.py::test_a[1]
  reports "no match" under pedantic/messy: the argument matcher compares the
  last part against the definition's bare name, and the parametrization lives on
  the items below it. Fixed by looking through the definition node; regression
  test covers 6 selector shapes × 3 modes.
• Skipping a non-Python item without a line number crashes pytest with an
  INTERNALERROR on assert line is not None in reports.py. Long-standing,
  but trivially reachable now that a non-Python definition can carry
  pytest.param(..., marks=pytest.mark.skip(...)). This one has no issue of its
  own; it is filed under the Opt-in FunctionDefinition collection tree node (#3926) #14769 changelog entry rather than under an invented
  number.

Notes for review

• HIGH_SCOPES now includes Definition, so reorder_items recurses one extra
  level on every run (O(n) per collection). Left as-is rather than special-cased.
• get_param_argkeys gained a scoped_definition_id key field and now reads
  item.cls defensively, since non-Python items can carry a callspec.
• 24 new tests in testing/test_definition_scope.py and
  testing/test_item_definition.py (the latter drives a real non-Python
  collector end to end), plus the selection matrix in
  testing/test_collect_function_definition.py.

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