WiFiManager: stream info and wifi pages through DFTE.

Harden the root placeholder path, add direct render-isolation coverage, move simple status pages and /info onto request-scoped streamed templates, and switch /wifi to a streamed shell while keeping scan/form sections string-built for now.

test/test_wifimanager/test_main.cpp

@@ -32,6 +32,7 @@ TestCase tests[] = {
     TEST_ENTRY(test_get_config_portal_ssid),
     TEST_ENTRY(test_root_render_menu_state_transitions),
     TEST_ENTRY(test_root_render_snapshot_consistency),
+    TEST_ENTRY(test_root_render_interleaved_context_isolation),
     
     // Non-blocking tests
     TEST_ENTRY(test_nonblocking_process),