Emit custom tool_result SSE event in Anthropic stream

Adds a non-standard tool_result event between the tool_use block close
and the next text block, so clients can see server-side tool execution
results. Anthropic SDKs ignore unknown event types.

studio/backend/core/inference/anthropic_compat.py

@@ -224,6 +224,12 @@ class AnthropicStreamEmitter:
         events = []
         # Close the tool_use block
         events.append(self._close_block())
+        # Emit custom tool_result event (non-standard, ignored by SDKs)
+        events.append(build_anthropic_sse_event("tool_result", {
+            "type": "tool_result",
+            "tool_use_id": event.get("tool_call_id", ""),
+            "content": event.get("result", ""),
+        }))
         # Open a new text block for the model's next response
         self.block_index += 1
         events.extend(self._open_text_block())

studio/backend/tests/test_anthropic_messages.py

@@ -328,11 +328,15 @@ class TestAnthropicStreamEmitter:
         e.start("msg_1", "m")
         e.feed({"type": "tool_start", "tool_name": "t", "tool_call_id": "tc_1", "arguments": {}})
         events = e.feed({"type": "tool_end", "tool_name": "t", "tool_call_id": "tc_1", "result": "done"})
-        # content_block_stop (tool) + content_block_start (new text)
-        assert len(events) == 2
+        # content_block_stop (tool) + tool_result + content_block_start (new text)
+        assert len(events) == 3
         assert "content_block_stop" in events[0]
-        assert "content_block_start" in events[1]
-        assert '"type": "text"' in events[1]
+        assert "tool_result" in events[1]
+        parsed = json.loads(events[1].split("data: ")[1])
+        assert parsed["content"] == "done"
+        assert parsed["tool_use_id"] == "tc_1"
+        assert "content_block_start" in events[2]
+        assert '"type": "text"' in events[2]
 
     def test_finish_emits_stop_events(self):
         e = AnthropicStreamEmitter()