http.DefaultTransport is shared with httptest.Server, which calls
CloseIdleConnections on it during Close. Parallel subtests sharing this
transport could see their in-flight requests broken with 'net/http:
HTTP/1.x transport connection broken: http: CloseIdleConnections
called'. Most visibly this flaked TestExternalAuthCallback/ValidateURL.
Lazily create a dedicated http.Client per OAuth2Config so its idle
connection pool is not affected by unrelated httptest.Server.Close
calls.
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Zach