Fix ExtensionEvent serialization/deserialization mismatch   (#2800)

## Overview
Fixes a `FormatException: Expected "ExtensionEvent" as first element` that occurs when deserializing `ExtensionEvents` inside `BatchedEvents`.

Unlike other types, `ExtensionEvents` sent inside `BatchedEvents` can be missing the `"ExtensionEvent"` type header and send `params` as a JSON Map Object instead of a String.

## Changes
- Modified `ExtensionEvent.fromJson` in `dwds/lib/data/extension_request.dart` to:
    - Handle missing headers by falling back to flat list parsing (meaning it assumes alternate indices as keys/values).
    - Handle `params` whether it is a `String` or a `Map`.