fix(json): absent attributes are dropped, so "every element has X" is vacuously true

The json provider calls providers/common.get_path_value_from_input with the default
place_none_if_not_found=False: elements lacking the path are dropped rather than padded, so a
document that omits the attribute entirely — the classic violation, e.g. a VM with no
linuxConfiguration block — reports null instead of failing. Any wildcard-path policy can catch
an explicitly wrong value but never an absent one.

Fix. Expose the flag on get_value (default keeps today's behaviour). The same code path is
needed by "type-scoped member reads on the json provider" (R2).

Acceptance. A fixture with the attribute absent fails when the flag is set; a fixture with the
attribute present and compliant still passes.

Start with the JSON provider's get_value entry point and providers/common.get_path_value_from_input, tracing how the place_none_if_not_found flag is passed. Add the absent-attribute fixture and verify it fails when the flag is enabled, while a present and compliant attribute fixture still passes.