django: a context bundle over the socket

The socket carried rpc but not ctx, so a socket transport could not fetch a
context and every app needed an HTTP connection beside it. Dispatches through
the same execute_context the HTTP view calls; that view's origin-side cache is
a CDN concern with nothing in front of a socket, so it is not on this path.

Both backends now answer subscribe/unsubscribe/message/rpc/ctx on one envelope.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backends/mizan-django/src/mizan/channels/connection.py

@@ -12,6 +12,7 @@ Protocol:
 
         # RPC calls (server functions)
         {"action": "rpc", "id": "request-id", "fn": "function_name", "args": {...}}
+        {"action": "ctx", "id": "request-id", "context": "name", "params": {...}}
 
     Server sends:
         # Channel messages
@@ -167,6 +168,8 @@ class DjangoReactConsumer(AsyncJsonWebsocketConsumer):
             await self._handle_message(content)
         elif action == "rpc":
             await self._handle_rpc(content)
+        elif action == "ctx":
+            await self._handle_ctx(content)
         else:
             await self.send_json(
                 {
@@ -503,6 +506,61 @@ class DjangoReactConsumer(AsyncJsonWebsocketConsumer):
 
             await self.send_json({"id": request_id, "ok": True, "data": data})
 
+    async def _handle_ctx(self, content: dict):
+        """
+        Handle a context-bundle fetch.
+
+        Protocol:
+            Request:  {"action": "ctx", "id": "request-id", "context": "name", "params": {...}}
+            Response: {"id": "request-id", "ok": true, "data": {fn_name: result, ...}}
+                  or: {"id": "request-id", "ok": false, "error": {...}}
+
+        Dispatches through the same execute_context the HTTP view calls. That view's
+        origin-side cache is a CDN concern with nothing in front of a socket, so it is not
+        part of this path.
+        """
+        from mizan.client.executor import execute_context, FunctionError
+
+        request_id = content.get("id")
+        context_name = content.get("context")
+
+        if not request_id:
+            await self.send_json({"error": "ctx request missing 'id' field"})
+            return
+
+        if not context_name:
+            await self.send_json(
+                {
+                    "id": request_id,
+                    "ok": False,
+                    "error": {"code": "BAD_REQUEST", "message": "Missing 'context' field"},
+                }
+            )
+            return
+
+        ws_request = WebSocketRequest(
+            self.scope, channel_name=getattr(self, "channel_name", None)
+        )
+        result = await sync_to_async(execute_context, thread_sensitive=True)(
+            ws_request, context_name, content.get("params") or {}
+        )
+
+        if isinstance(result, FunctionError):
+            await self.send_json(
+                {
+                    "id": request_id,
+                    "ok": False,
+                    "error": {
+                        "code": result.code.value,
+                        "message": result.message,
+                        **({"details": result.details} if result.details else {}),
+                    },
+                }
+            )
+            return
+
+        await self.send_json({"id": request_id, "ok": True, "data": result.data})
+
     async def channel_message(self, event: dict):
         """
         Handle messages broadcast to a group.