fix(openai_dart): Fetch requests with big payloads dropping connection

[davidmigloz]

The fetch_client that openai_dart uses when targeting web maps keepalive to BaseRequest.persistentConnection
which is true by default.

Fetch spec says that maximum request size with keepalive flag is 64KiB:

4.5. HTTP-network-or-cache fetch

8.10.5: If the sum of contentLength and inflightKeepaliveBytes is greater
than 64 kibibytes, then return a network error.

Source: Fetch. Living Standard — Last Updated 19 June 2023

Therefore if the request is larger than 64KiB (this includes some other data, so effectively recommended check is 60KiB) we must explicitly set BaseRequest.persistentConnection
to false, otherwise request will fail.