feat: add file handler e2e tests covering GET, PUT, index, auth, and 404

Adds 5 test methods (7 test cases) for the file handler:
GET existing file, GET nonexistent file, GET index.html, PUT upload,
PUT without permission, and auth (no-auth-401, with-auth-success).

Includes test data files and server YAML configs under
tests/e2e/testdata/file/.

tests/e2e/testdata/file/server_auth.yaml

@@ -0,0 +1,15 @@
+services:
+- name: file-server-auth
+  addr: :8080
+  handler:
+    type: file
+    auther: auther-0
+    metadata:
+      file.dir: /srv/files
+  listener:
+    type: tcp
+authers:
+- name: auther-0
+  auths:
+  - username: user
+    password: pass