It's too fragile to put in side core RPC machinery since
`msgspec.Struct` defs can fail if a field type can't be
looked up at creation time (like can easily happen if you
conditionally import using `if TYPE_CHECKING:`)
Also,
- rename `cs` to `rpc_ctx_cs: CancelScope` since it's literally
the wrapping RPC `Context._scope`.
- report self cancellation via `explain: str` and add tail case for
"unknown cause".
- put a ?TODO? around what to do about KBIs if a context is opened
from an `infected_aio`-actor task.
- similar to our nursery and portal add TODO list for moving all
`_invoke_non_context()` content out the RPC core and instead implement
them as `.hilevel` endpoint helpers (maybe as decorators?)which under
neath define `@context`-funcs.
7e93b81a83