refactor(gateway): shared reaction-ack policy in base adapter

- base.on_processing_complete implements the opt-in remove-ack/add-outcome
  flow driven by _OK_EMOJI/_FAIL_EMOJI class attrs and the
  _add_reaction(chat_id, message_id, emoji)/_remove_reaction(chat_id,
  message_id) primitive shape; default stays a no-op.
- photon drops its override (exact behavioral match).
- slack/discord/feishu/matrix/telegram/google_chat keep overrides: divergent
  primitive signatures (team_id routing, raw message objects, reaction-id
  handles, replace-semantics setMessageReaction) or extra state protocols.

gateway/platforms/base.py

@@ -4793,11 +4793,52 @@ class BasePlatformAdapter(ABC):
     # Subclasses override these to react to message processing events
     # (e.g. Discord adds 👀/✅/❌ reactions).
 
+    # Opt-in emoji set for the shared reaction-ack flow in
+    # ``on_processing_complete``. Adapters whose reaction primitives follow
+    # the ``_add_reaction(chat_id, message_id, emoji)`` /
+    # ``_remove_reaction(chat_id, message_id)`` shape can set these class
+    # attributes instead of overriding the hook. Left as ``None`` the hook
+    # stays a no-op (historical default).
+    _ACK_EMOJI: Optional[str] = None
+    _OK_EMOJI: Optional[str] = None
+    _FAIL_EMOJI: Optional[str] = None
+
     async def on_processing_start(self, event: MessageEvent) -> None:
         """Hook called when background processing begins."""
 
     async def on_processing_complete(self, event: MessageEvent, outcome: ProcessingOutcome) -> None:
-        """Hook called when background processing completes."""
+        """Hook called when background processing completes.
+
+        Default: shared reaction-ack flow — swap the in-progress reaction for
+        a final success/failure reaction. Runs only when the adapter opts in
+        by setting ``_OK_EMOJI`` / ``_FAIL_EMOJI`` class attributes AND
+        defines ``_add_reaction`` / ``_remove_reaction`` primitives taking
+        ``(chat_id, message_id[, emoji])``. Otherwise this is a no-op, as it
+        always was. Remove-then-add rather than a bare replace: deterministic
+        whether the platform replaces a sender's previous reaction or stacks
+        them. CANCELLED outcomes leave the message unreacted.
+        """
+        if self._OK_EMOJI is None and self._FAIL_EMOJI is None:
+            return
+        add: Any = getattr(self, "_add_reaction", None)
+        remove: Any = getattr(self, "_remove_reaction", None)
+        if not callable(add) or not callable(remove):
+            return
+        enabled = getattr(self, "_reactions_enabled", None)
+        if callable(enabled) and not enabled():
+            return
+        chat_id = getattr(event.source, "chat_id", None)
+        message_id = getattr(event, "message_id", None)
+        if not chat_id or not message_id:
+            return
+        await remove(chat_id, message_id)
+        if outcome == ProcessingOutcome.SUCCESS:
+            if self._OK_EMOJI:
+                await add(chat_id, message_id, self._OK_EMOJI)
+        elif outcome == ProcessingOutcome.FAILURE:
+            if self._FAIL_EMOJI:
+                await add(chat_id, message_id, self._FAIL_EMOJI)
+        # CANCELLED: leave the message unreacted.
 
     async def _run_processing_hook(self, hook_name: str, *args: Any, **kwargs: Any) -> None:
         """Run a lifecycle hook without letting failures break message flow."""

plugins/platforms/photon/adapter.py

@@ -2295,27 +2295,13 @@ class PhotonAdapter(BasePlatformAdapter):
         if chat_id and message_id:
             await self._add_reaction(chat_id, message_id, "\U0001f440")
 
-    async def on_processing_complete(
-        self, event: MessageEvent, outcome: ProcessingOutcome
-    ) -> None:
-        """Swap the 👀 progress tapback for a 👍/👎 result.
-
-        Remove-then-add rather than a bare replace: deterministic whether the
-        platform replaces a sender's previous tapback or stacks them, and it
-        keeps the sidecar's reaction-handle slot coherent.
-        """
-        if not self._reactions_enabled():
-            return
-        chat_id = getattr(event.source, "chat_id", None)
-        message_id = getattr(event, "message_id", None)
-        if not chat_id or not message_id:
-            return
-        await self._remove_reaction(chat_id, message_id)
-        if outcome == ProcessingOutcome.SUCCESS:
-            await self._add_reaction(chat_id, message_id, "\U0001f44d")
-        elif outcome == ProcessingOutcome.FAILURE:
-            await self._add_reaction(chat_id, message_id, "\U0001f44e")
-        # CANCELLED: leave the message unreacted.
+    # Shared reaction-ack flow (base.on_processing_complete): swap the 👀
+    # progress tapback for a 👍/👎 result. Remove-then-add rather than a bare
+    # replace: deterministic whether the platform replaces a sender's previous
+    # tapback or stacks them, and it keeps the sidecar's reaction-handle slot
+    # coherent. CANCELLED: leave the message unreacted.
+    _OK_EMOJI = "\U0001f44d"
+    _FAIL_EMOJI = "\U0001f44e"
 
     async def get_chat_info(self, chat_id: str) -> Dict[str, Any]:
         """Return whatever we know about a Spectrum space id.