Update pld-rx limiting test(s) to use deco input

The tests only use one input spec (conveniently) so there's not much to
change in the logic,
- only pass the `maybe_msg_spec` to the child-side decorator and obvi
  drop the surrounding `msgops.limit_plds()` block in the child.
- tweak a few `MsgDec` asserts, mostly dropping the
  `msg._ops._def_any_spec` state checks since the child-side won't have
  any pre pld-spec state given the runtime now applies the `pld_spec`
  before running the task's func body.
  - also allowed dropping the `finally:` which did a similar check
    outside the `.limit_plds()` block.

tests/test_pldrx_limiting.py

@@ -7,9 +7,6 @@ related settings around IPC contexts.
 from contextlib import (
     asynccontextmanager as acm,
 )
-from contextvars import (
-    Context,
-)
 
 from msgspec import (
     Struct,
@@ -19,6 +16,7 @@ import trio
 
 import tractor
 from tractor import (
+    Context,
     MsgTypeError,
     current_ipc_ctx,
     Portal,
@@ -35,7 +33,17 @@ from tractor.msg.types import (
 )
 
 
-class PldMsg(Struct):
+class PldMsg(
+    Struct,
+
+    # TODO: with multiple structs in-spec we need to tag them!
+    # -[ ] offer a built-in `PldMsg` type to inherit from which takes
+    #      case of these details?
+    #
+    # https://jcristharif.com/msgspec/structs.html#tagged-unions
+    # tag=True,
+    # tag_field='msg_type',
+):
     field: str
 
 
@@ -96,7 +104,9 @@ async def maybe_expect_raises(
                 )
 
 
-@tractor.context
+@tractor.context(
+    pld_spec=maybe_msg_spec,
+)
 async def child(
     ctx: Context,
     started_value: int|PldMsg|None,
@@ -116,21 +126,19 @@ async def child(
     assert ctx is curr_ctx
 
     rx: msgops.PldRx = ctx._pld_rx
-    orig_pldec: _codec.MsgDec = rx.pld_dec
-    # senity that default pld-spec should be set
-    assert (
-        rx.pld_dec
-        is
-        msgops._def_any_pldec
-    )
+    curr_pldec: _codec.MsgDec = rx.pld_dec
 
-    try:
-        with msgops.limit_plds(
-            spec=maybe_msg_spec,
-        ) as pldec:
-            # sanity on `MsgDec` state
-            assert rx.pld_dec is pldec
-            assert pldec.spec is maybe_msg_spec
+    ctx_meta: dict = getattr(
+        child,
+        '_tractor_context_meta',
+        None,
+    )
+    if ctx_meta:
+        assert (
+            ctx_meta['pld_spec']
+            is curr_pldec.spec
+            is curr_pldec.pld_spec
+        )
 
     # 2 cases: hdndle send-side and recv-only validation
     # - when `raise_on_started_mte == True`, send validate
@@ -211,18 +219,6 @@ async def child(
     # msg-type-error from this RPC task ;)
     return return_value
 
-    finally:
-        # sanity on `limit_plds()` reversion
-        assert (
-            rx.pld_dec
-            is
-            msgops._def_any_pldec
-        )
-        log.runtime(
-            'Reverted to previous pld-spec\n\n'
-            f'{orig_pldec}\n'
-        )
-
 
 @pytest.mark.parametrize(
     'return_value',
@@ -321,7 +317,6 @@ def test_basic_payload_spec(
                     child,
                     return_value=return_value,
                     started_value=started_value,
-                    pld_spec=maybe_msg_spec,
                     validate_pld_spec=pld_check_started_value,
                 ) as (ctx, first),
             ):