refactor(sandbox): share container-path utils and tighten fs bridge tests

src/agents/sandbox/path-utils.ts

@@ -0,0 +1,15 @@
+import path from "node:path";
+
+export function normalizeContainerPath(value: string): string {
+  const normalized = path.posix.normalize(value);
+  return normalized === "." ? "/" : normalized;
+}
+
+export function isPathInsideContainerRoot(root: string, target: string): boolean {
+  const normalizedRoot = normalizeContainerPath(root);
+  const normalizedTarget = normalizeContainerPath(target);
+  if (normalizedRoot === "/") {
+    return true;
+  }
+  return normalizedTarget === normalizedRoot || normalizedTarget.startsWith(`${normalizedRoot}/`);
+}