No check that oracle validator set size >= minimum before median calculation

[Junman140]

acbu_oracle/src/lib.rs: update_rate computes the median of the current epoch's submissions. If only 1 validator submits (others offline), the "median" is a single potentially malicious rate with no safety net.

[hartz0]

@hartz0 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to solve this issue please , thank you

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[drips-wave]

Congratulations, @hartz0! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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