arsdk: read the codec's (ids, values) decode shape

protocol/codec.py landed with decode_event returning ids and a values
dict, so the session looks the command name up in the index rather than
expecting it in the result. Verified end to end against that stream's
codec in a stitched tree: the sim encodes events through it, the session
decodes through it, and the test that skips until it exists now passes.

The argument name cannot be recovered by splitting a telemetry key,
because an argument can contain an underscore, so the expectation
predicate matches on the suffix instead.

src/mcbebop/arsdk/session.py

@@ -24,6 +24,7 @@ import threading
 import time
 from collections.abc import Callable, Iterable
 from dataclasses import dataclass, field
+from functools import cache
 from typing import Any
 
 from mcbebop.arsdk.connection import Connection
@@ -101,14 +102,31 @@ def _default_decoder(payload: bytes) -> Any:
     return decode_event(payload)
 
 
+@cache
+def _name_for(ids: tuple[int, int, int]) -> str:
+    """The command's own name, for an event that arrived without one.
+
+    `protocol.codec.decode_event` returns ids and values; the name lives in
+    the index. It cannot be recovered from the keys, because an argument may
+    itself contain an underscore (`MassStorageInfoStateListChanged_mass_storage_id`).
+    """
+    try:
+        from mcbebop.protocol import xml_index  # deferred, and optional
+
+        spec = xml_index.index().by_ids(ids)
+    except Exception:  # pragma: no cover - only without the protocol layer
+        spec = None
+    return spec.name if spec is not None else f"cmd_{ids[0]}_{ids[1]}_{ids[2]}"
+
+
 def _as_event(decoded: Any, payload: bytes) -> Event | None:
     """Normalise whatever the codec returned into an `Event`.
 
-    The codec is written by another stream against the same `Event` type, so
+    `decode_event` returns `(ids, values)`, which is the second branch. The
-    the first branch is the expected one. The rest exist because a decoder
+    others are here because an `Event`, a 3-tuple and a mapping are all
-    that returns a plain tuple or mapping is a reasonable thing to write, and
+    reasonable things for a decoder or a test fake to hand back, and a
-    a transport that crashes on the shape of a decode result would be a silly
+    transport that died on the shape of a decode result would be a silly way
-    way to lose a link.
+    to lose a link.
     """
     if decoded is None:
         return None
@@ -119,16 +137,18 @@ def _as_event(decoded: Any, payload: bytes) -> Event | None:
     values = getattr(decoded, "values", None)
     if ids is None and isinstance(decoded, dict):
         ids, name, values = decoded.get("ids"), decoded.get("name"), decoded.get("values")
-    if ids is None and isinstance(decoded, tuple | list) and len(decoded) == 3:
+    if ids is None and isinstance(decoded, tuple | list):
-        ids, name, values = decoded
+        if len(decoded) == 2:
+            ids, values = decoded
+        elif len(decoded) == 3:
+            ids, name, values = decoded
     if ids is None:
         log.debug("decoder returned an unusable shape %r for %s", type(decoded), payload[:8].hex())
         return None
-    if name is None:
+    ids = tuple(ids)
-        name = f"cmd_{tuple(ids)}"
     return Event(
-        ids=tuple(ids),  # type: ignore[arg-type]
+        ids=ids,  # type: ignore[arg-type]
-        name=str(name),
+        name=str(name) if name else _name_for(ids),  # type: ignore[arg-type]
         values=dict(values or {}),
         at=getattr(decoded, "at", None) or time.monotonic(),
     )
@@ -409,7 +429,13 @@ class DroneSession:
 
         def predicate(event: Event) -> bool:
             for name, expected in checks.get(event.ids, {}).items():
-                got = event.values.get(f"{event.name}_{name}", event.values.get(name))
+                got = event.values.get(f"{event.name}_{name}")
+                if got is None:
+                    # Telemetry keys are `<Command>_<arg>`, but an argument
+                    # can contain an underscore, so match on the suffix
+                    # rather than trusting a split.
+                    matches = [v for k, v in event.values.items() if k.endswith(f"_{name}") or k == name]
+                    got = matches[0] if len(matches) == 1 else None
                 if got is not None and got != expected:
                     return False
             return True