Parameterize the `return_msg_type` in `._invoke()`

Since we also handle a runtime-specific `CancelAck`, allow the
caller-scheduler to pass in the expected return-type msg per the RPC msg
endpoint loop.

tractor/_rpc.py

@@ -63,6 +63,7 @@ from .log import get_logger
 from .msg import (
     current_codec,
     MsgCodec,
+    PayloadT,
     NamespacePath,
     pretty_struct,
 )
@@ -96,7 +97,7 @@ async def _invoke_non_context(
 
     treat_as_gen: bool,
     is_rpc: bool,
-    return_msg: Return|CancelAck = Return,
+    return_msg_type: Return|CancelAck = Return,
 
     task_status: TaskStatus[
         Context | BaseException
@@ -218,7 +219,7 @@ async def _invoke_non_context(
                 and chan.connected()
             ):
                 try:
-                    ret_msg = return_msg(
+                    ret_msg = return_msg_type(
                         cid=cid,
                         pld=result,
                     )
@@ -417,7 +418,7 @@ async def _invoke(
 
     is_rpc: bool = True,
     hide_tb: bool = True,
-    return_msg: Return|CancelAck = Return,
+    return_msg_type: Return|CancelAck = Return,
 
     task_status: TaskStatus[
         Context | BaseException
@@ -531,7 +532,7 @@ async def _invoke(
                 kwargs,
                 treat_as_gen,
                 is_rpc,
-                return_msg,
+                return_msg_type,
                 task_status,
             )
             # XXX below fallthrough is ONLY for `@context` eps
@@ -591,18 +592,21 @@ async def _invoke(
                 ctx._scope = tn.cancel_scope
                 task_status.started(ctx)
 
-                # TODO: should would be nice to have our
-                # `TaskMngr` nursery here!
-                res: Any = await coro
-                ctx._result = res
-
-                # deliver final result to caller side.
-                await chan.send(
-                    return_msg(
+                # TODO: better `trionics` tooling:
+                # -[ ] should would be nice to have our `TaskMngr`
+                #   nursery here!
+                # -[ ] payload value checking like we do with
+                #   `.started()` such that the debbuger can engage
+                #   here in the child task instead of waiting for the
+                #   parent to crash with it's own MTE..
+                res: Any|PayloadT = await coro
+                return_msg: Return|CancelAck = return_msg_type(
                     cid=cid,
                     pld=res,
                 )
-                )
+                # set and shuttle final result to "parent"-side task.
+                ctx._result = res
+                await chan.send(return_msg)
 
             # NOTE: this happens IFF `ctx._scope.cancel()` is
             # called by any of,
@@ -938,7 +942,7 @@ async def process_messages(
                                 actor.cancel,
                                 kwargs,
                                 is_rpc=False,
-                                return_msg=CancelAck,
+                                return_msg_type=CancelAck,
                             )
 
                         log.runtime(
@@ -972,7 +976,7 @@ async def process_messages(
                                 actor._cancel_task,
                                 kwargs,
                                 is_rpc=False,
-                                return_msg=CancelAck,
+                                return_msg_type=CancelAck,
                             )
                         except BaseException:
                             log.exception(