Built-in adapters (Signal, WhatsApp Cloud, Weixin, MSGraph, BlueBubbles, ...)
were returned from the if/elif factory without `gateway_runner`, so
`build_source` never consulted `profile_routes` for them — routed inbound
events landed in the default profile's agent:main namespace. Only the
plugin-registry branch and api_server/webhook set the back-reference.
Split the factory: `_instantiate_adapter` builds, `_create_adapter` binds
the runner on every non-None result. All lifecycle callers (primary
startup, reconnect, secondary-profile startup) already go through
`_create_adapter`, so this covers every path with one seam instead of
per-branch assignments.
Salvaged from #70831 (Hudson). First reported in #68332.
db639e1023