fix(gno): use first present precommit for Commit height/round instead of index 0

[aronpark1007]

Summary

• Commit::height() / Commit::round() only ever looked at precommits[0], defaulting to 0 whenever that specific validator hadn't signed the block.
• This caused the height check to be compared against a wrong (zero) value, which surfaced as an intermittent failure depending on which validators had signed.

Changes

• Find the first present precommit instead of always indexing [0], so height/round are read from a validator that actually signed.

Test plan

• Verified on union-devnet after migrating the gno light client contract with this fix — confirmed working on-chain.