fix(genesis): require positive validator voting power

[sanjeebdas1979]

Summary

Require genesis validator configurations to include at least one validator with positive voting power.

The previous schema accepted both an empty validator set and a set where every validator had zero voting power. Consensus-side decoding later rejects the resulting empty effective validator set, so this validation moves the failure earlier to genesis configuration time.

Changes

• Add set-level validation requiring at least one validator with positive voting power.
• Preserve support for zero-power validators when another validator has positive voting power.
• Add regression tests covering:
  • empty validator set
  • all-zero voting power
  • positive-only validator set
  • mixed zero/positive validator set

Testing

ValidatorManager genesis validator-set validation
  ✔ rejects an empty validator set
  ✔ rejects a validator set with no positive voting power
  ✔ accepts a validator set with positive voting power
  ✔ accepts zero-power validators when another validator has positive power

4 passing

Fixes #344