Fix Windows integration test for background jobs

Replace `&&` with `;` in the bg-shell mock command. PowerShell does
not support `&&` as a statement separator, causing the background job
to fail immediately on Windows CI. Semicolons work in both bash and
PowerShell.

🤖 Generated with [eca](https://eca.dev)

Co-Authored-By: eca-agent <git@eca.dev>

Author: ericdallo

integration-test/llm_mock/anthropic.clj

@@ -318,7 +318,7 @@
                    {:type "content_block_delta"
                     :index 1
                     :delta {:type "input_json_delta"
-                            :partial_json "{\"command\":\"echo bg-test-output && sleep 30\",\"background\":\"bg-test\"}"}})
+                            :partial_json "{\"command\":\"echo bg-test-output; sleep 30\",\"background\":\"bg-test\"}"}})
         (sse-send! ch "message_delta"
                    {:type "message_delta"
                     :delta {:stop_reason "tool_use"}