feat(discord): add message processing reactions (salvage #1980) (#3871)

Adds lifecycle hooks to the base platform adapter so Discord (and future
platforms) can react to message processing events:

  👀  when processing starts
  ✅  on successful completion (delivery confirmed)
  ❌  on failure, error, or cancellation

Implementation:
- base.py: on_processing_start/on_processing_complete hooks with
  _run_processing_hook error isolation wrapper; delivery tracking
  via _record_delivery closure for accurate success detection
- discord.py: _add_reaction/_remove_reaction helpers + hook overrides
- Tests for base hook lifecycle and Discord-specific reactions

Co-authored-by: alanwilhelm <alanwilhelm@users.noreply.github.com>

gateway/platforms/base.py

@@ -898,6 +898,26 @@ class BasePlatformAdapter(ABC):
                 except Exception:
                     pass
     
+    # ── Processing lifecycle hooks ──────────────────────────────────────────
+    # Subclasses override these to react to message processing events
+    # (e.g. Discord adds 👀/✅/❌ reactions).
+
+    async def on_processing_start(self, event: MessageEvent) -> None:
+        """Hook called when background processing begins."""
+
+    async def on_processing_complete(self, event: MessageEvent, success: bool) -> None:
+        """Hook called when background processing completes."""
+
+    async def _run_processing_hook(self, hook_name: str, *args: Any, **kwargs: Any) -> None:
+        """Run a lifecycle hook without letting failures break message flow."""
+        hook = getattr(self, hook_name, None)
+        if not callable(hook):
+            return
+        try:
+            await hook(*args, **kwargs)
+        except Exception as e:
+            logger.warning("[%s] %s hook failed: %s", self.name, hook_name, e)
+
     @staticmethod
     def _is_retryable_error(error: Optional[str]) -> bool:
         """Return True if the error string looks like a transient network failure."""
@@ -1060,6 +1080,18 @@ class BasePlatformAdapter(ABC):
 
     async def _process_message_background(self, event: MessageEvent, session_key: str) -> None:
         """Background task that actually processes the message."""
+        # Track delivery outcomes for the processing-complete hook
+        delivery_attempted = False
+        delivery_succeeded = False
+
+        def _record_delivery(result):
+            nonlocal delivery_attempted, delivery_succeeded
+            if result is None:
+                return
+            delivery_attempted = True
+            if getattr(result, "success", False):
+                delivery_succeeded = True
+
         # Create interrupt event for this session
         interrupt_event = asyncio.Event()
         self._active_sessions[session_key] = interrupt_event
@@ -1069,6 +1101,8 @@ class BasePlatformAdapter(ABC):
         typing_task = asyncio.create_task(self._keep_typing(event.source.chat_id, metadata=_thread_metadata))
         
         try:
+            await self._run_processing_hook("on_processing_start", event)
+
             # Call the handler (this can take a while with tool calls)
             response = await self._message_handler(event)
             
@@ -1138,6 +1172,7 @@ class BasePlatformAdapter(ABC):
                         reply_to=event.message_id,
                         metadata=_thread_metadata,
                     )
+                    _record_delivery(result)
 
                 # Human-like pacing delay between text and media
                 human_delay = self._get_human_delay()
@@ -1237,6 +1272,10 @@ class BasePlatformAdapter(ABC):
                     except Exception as file_err:
                         logger.error("[%s] Error sending local file %s: %s", self.name, file_path, file_err)
 
+            # Determine overall success for the processing hook
+            processing_ok = delivery_succeeded if delivery_attempted else not bool(response)
+            await self._run_processing_hook("on_processing_complete", event, processing_ok)
+
             # Check if there's a pending message that was queued during our processing
             if session_key in self._pending_messages:
                 pending_event = self._pending_messages.pop(session_key)
@@ -1253,7 +1292,11 @@ class BasePlatformAdapter(ABC):
                 await self._process_message_background(pending_event, session_key)
                 return  # Already cleaned up
                 
+        except asyncio.CancelledError:
+            await self._run_processing_hook("on_processing_complete", event, False)
+            raise
         except Exception as e:
+            await self._run_processing_hook("on_processing_complete", event, False)
             logger.error("[%s] Error handling message: %s", self.name, e, exc_info=True)
             # Send the error to the user so they aren't left with radio silence
             try: