feat: grep_files as a tool (#4820)

Add `grep_files` to be able to perform more action in parallel

codex-rs/core/tests/suite/tool_parallelism.rs

@@ -63,8 +63,9 @@ async fn build_codex_with_test_tool(server: &wiremock::MockServer) -> anyhow::Re
 }
 
 fn assert_parallel_duration(actual: Duration) {
+    // Allow headroom for runtime overhead while still differentiating from serial execution.
     assert!(
-        actual < Duration::from_millis(500),
+        actual < Duration::from_millis(750),
         "expected parallel execution to finish quickly, got {actual:?}"
     );
 }