fix: preserve existing refresh_token when server omits it in refresh response

[ctonneslan]

Problem

_handle_refresh_response() replaces current_tokens with the parsed refresh response as-is. When the authorization server does not return a new refresh_token in the response, the previously stored one is lost.

After the first successful refresh, can_refresh_token() returns False and all subsequent refreshes fail, forcing full re-authentication.

Per RFC 6749 Section 6, issuing a new refresh token in the refresh response is optional. Many OAuth providers omit it by default (Google, Auth0 without rotation enabled, Okta in persistent token mode).

Fix

Before overwriting current_tokens, check if the refresh response omits refresh_token. If so, preserve the existing one using model_copy(update=...).

if (
    not token_response.refresh_token
    and self.context.current_tokens
    and self.context.current_tokens.refresh_token
):
    token_response = token_response.model_copy(
        update={"refresh_token": self.context.current_tokens.refresh_token}
    )

This matches the RFC wording: if a new refresh token is issued, replace the old one; otherwise keep using the existing one.

Fixes #2270