bug: OpenAI pass though routes are forwarded with double `/v1` prefix

[pawbana]

With default OpenAI base url https://api.openai.com/v1/ pass though routes have v1 prefix added 2 times, once from base URL (added in #159)

aibridge/passthrough.go

Line 40 in aed7dd9

reqPath, err := url.JoinPath(upURL.Path, r.URL.Path)

and then from r.URL.Path which contains /v1 prefix

aibridge/provider/openai.go

Lines 76 to 80 in aed7dd9

	"/v1/conversations",
	"/v1/conversations/",
	"/v1/models",
	"/v1/models/",
	"/v1/responses/", // Forwards other responses API endpoints, eg: https://platform.openai.com/docs/api-reference/responses/get

that isn't stripped

aibridge/bridge.go

Line 104 in aed7dd9

mux.HandleFunc(route, http.StripPrefix(prefix, ftr).ServeHTTP)