fix: oauth token race in http_transport

[burmudar]

Fix a read race mentioned here by @keegancsmith

Test plan

Unit tests

[keegancsmith]

very minor: you lock in refresh and in getToken, and this is the only place you call both. Maybe instead you can refactor it to read like this

Suggested change

	if err := t.refreshToken(ctx); err != nil {
	return nil, err
	}
	token := t.getToken()
	token, err := t.getToken(ctx)
	if err != nil {
	return nil, err
	}

[burmudar]

yeah I tend to agree. It did feel bit 💩 to do it - only thing that stopped me was getToken does more than what it says, although it's minor in the long run.

I'll update it in a bit.