_bridge_env copied os.environ (the default profile's WHATSAPP_* values under
multiplex) and only overlaid scoped hits, so a secondary with YAML
`dm_policy: pairing` launched its Node bridge under the default profile's
`allowlist` policy and the bridge rejected valid pairing DMs before Python saw
them. The child env now carries the values the adapter resolved (scoped env →
own YAML → default); a scoped miss removes the key rather than inheriting it.
Refs #108440 (ehz0ah inline, plugins/platforms/whatsapp/adapter.py)