fix(consensus): guard against zero total stake in VRF validator selection

[HusseinAdeiza]

Summary

Closes #1732. get_validators_for_transaction divided each validator's stake by the total stake with no zero check. When all validators have stake=0 (e.g. the internally-created validators in protocol_rpc/endpoints.py), this raised ZeroDivisionError and crashed the consensus worker, halting transaction processing.

Change

Fall back to a uniform probability distribution when total_stake == 0; non-zero totals are unaffected. Adds a unit test (test_get_validators_for_transaction_all_zero_stake) covering the zero-stake path.

Testing

• New unit test covers the zero-stake → uniform path.
• Verified the function logic in isolation (the pre-existing vrf↔base circular import blocks importing vrf first, on pristine main too): zero-stake → [1/3,1/3,1/3], normal stakes → [1/6,2/6,3/6], mixed → stake weights.

Summary by CodeRabbit

• Bug Fixes
  • Validator selection now assigns equal probabilities to all validators when their combined stake is zero. When the combined stake is greater than zero, selection continues to use stake-proportional probabilities. This ensures that validators with zero stake can still be selected fairly when no stake is available to guide the selection.

[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v0.123-dev, the active development branch. Pushes to v0.123-dev automatically fast-forward main.

[coderabbitai]

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[HusseinAdeiza]

Closing in favor of the upstream fix that landed on v0.123-dev: get_validators_for_transaction now guards total_stake <= 0 (raises NoValidatorsAvailableError). Same root cause, maintainer-preferred fail-loud approach. Thanks.