test(app-server): add findFilesStream integration scenarios

Cover 10 scenarios: single match; empty query; empty roots; no matches; multi-root merge; same token updates request id/query; same token + different roots cancels old stream; per-root limit; chunking over chunk size; sorted unique indices.

Also fix a stream completion race by tracking walker liveness in codex-file-search’s DebouncedSearchManager/SearchManager.

codex-rs/app-server/tests/common/mcp_process.rs

@@ -573,6 +573,23 @@ impl McpProcess {
         self.send_request("fuzzyFileSearch", Some(params)).await
     }
 
+    /// Send a `findFilesStream` JSON-RPC request.
+    pub async fn send_find_files_stream_request(
+        &mut self,
+        query: &str,
+        roots: Vec<String>,
+        cancellation_token: Option<String>,
+    ) -> anyhow::Result<i64> {
+        let mut params = serde_json::json!({
+            "query": query,
+            "roots": roots,
+        });
+        if let Some(token) = cancellation_token {
+            params["cancellationToken"] = serde_json::json!(token);
+        }
+        self.send_request("findFilesStream", Some(params)).await
+    }
+
     async fn send_request(
         &mut self,
         method: &str,