GetOAuthFlow was the only flow factory without a hint parameter, which
forced its callers to apply the hint afterwards through a local setter
interface and a type assertion. Give it the same constructor-style hint
as NewOAuthFlow and set the hint on the concrete flows before they are
handed out as the interface, so a flow is always complete when built
and the caller-side ordering constraint disappears.
An empty hint is a valid value meaning the IdP chooses the account, so
the flows set it unconditionally.
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