webdriver: Properly insert the user agent into the capabilities data structure (#37876)

Add user agent into webdriver capabilities.

Testing:
`/mach test-wpt -r --product servodriver
./tests/wpt/tests/webdriver/tests/classic/new_session/response.py`

Signed-off-by: batu_hoang <longvatrong111@gmail.com>

components/webdriver_server/lib.rs

@@ -678,6 +678,10 @@ impl Handler {
                         "setWindowRect".to_string(),
                         json!(servo_capabilities.set_window_rect),
                     );
+                    processed.insert(
+                        "userAgent".to_string(),
+                        servo_config::pref!(user_agent).into(),
+                    );
 
                     let response =
                         NewSessionResponse::new(session.id.to_string(), Value::Object(processed));

tests/wpt/meta/webdriver/tests/classic/new_session/response.py.ini

@@ -1,3 +0,0 @@
-[response.py]
-  [test_capability_user_agent]
-    expected: FAIL