fix: resolve all call-non-callable ty diagnostics across codebase

Replace hasattr() duck-typing with isinstance() checks for DiscordAdapter
in gateway/run.py, add TypedDict for IMAGEGEN_BACKENDS in tools_config.py,
properly type fal_client getattr'd callables in image_generation_tool.py,
fix dict[str, object] → Callable annotation in approval.py, use
isinstance(BaseModel) in web_tools.py, capture _message_handler to local
in base.py, rename shadowed list_distributions parameter in batch_runner.py,
and remove dead queue_message branch.

gateway/platforms/base.py

@@ -1828,8 +1828,11 @@ class BasePlatformAdapter(ABC):
         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)
+            handler = self._message_handler
+            if handler is None:
+                return
+
+            response = await handler(event)
             
             # Send response if any.  A None/empty response is normal when
             # streaming already delivered the text (already_sent=True) or