fix: bounds check in GateSeparatorPolynomial for virtual sumcheck rounds

[AztecBot]

Summary

Ports the fix from #22130 (targeting next) to merge-train/barretenberg.

GateSeparatorPolynomial::current_element() and univariate_eval() accessed betas[current_element_idx] without bounds checking. During virtual sumcheck rounds (when current_element_idx >= betas.size()), this read out-of-bounds heap memory, causing non-deterministic debug build assertion failures:

Assertion failed: (val < twice_modulus)
Reason: field element exceeds coarse form [0, 2p)

The fix returns FF(1) when past the betas array, which is mathematically correct since virtual rounds have no additional beta challenges.

Full analysis: https://gist.github.com/AztecBot/909e3998f506b0dd6d07a211841dbeb8

ClaudeBox log: https://claudebox.work/s/39dda16ff88b644f?run=1

[AztecBot]

Automatically closing this stale claudebox draft PR (no updates for 5+ days). Re-open if still needed.