Elixir client `Message.from_message/2` eats change info.

When consuming a shape in an Elixir application using the Electric.Client, the parsing of raw messages into `Electric.Client.Message.ChangeMessage` structs loses the information about which fields have changed.

To reproduce, make a surgical update, e.g.: just updating the name on a schema with additional fields:

```elixir
Threads.change_thread(thread, %{name: "some other name"})
```

This results in a raw message like this:

```elixir
%{
"headers" => %{
"last" => true,
"lsn" => "786675360",
"op_position" => 0,
"operation" => "update",
"relation" => ["public", "threads"],
"txids" => [39380]
},
"key" => "\"public\".\"threads\"/\"c946e731-ddd7-4331-8147-6a159fede21f\"",
"value" => %{
"id" => "c946e731-ddd7-4331-8147-6a159fede21f",
"name" => "some other name"
}
}
```

This is processed by `ChangeMessage.from_message` (using `Electric.Client.EctoAdapter.for_schema/2`) into:

```elixir
%Electric.Client.Message.ChangeMessage{
key: "\"public\".\"threads\"/\"c946e731-ddd7-4331-8147-6a159fede21f\"",
value: %MyApp.Threads.Thread{
__meta__: #Ecto.Schema.Metadata<:built, "threads">,
id: "c946e731-ddd7-4331-8147-6a159fede21f",
name: "some other name",
status: nil,
inserted_at: nil,
updated_at: nil
},
old_value: nil,
headers: %Electric.Client.Message.Headers{
operation: :update,
relation: ["public", "threads"],
handle: "8728556-1749895640933410"
},
request_timestamp: ~U[2025-06-14 10:07:20.967291Z]
}
```

Note the addition of `status: nil, inserted_at: nil, updated_at: nil`. It's impossible to know whether these values were set to `nil` by the change or were not set.

It is possible to workaround this by setting `?replica=full` to get the old_value and then figuring out the changes. However, this seems unnecessary given that the information is present in the raw message already.