[v1.128.2] Add signature count validation in TSS keysign - mainnet - #check-lint-warning

Summary

Port security patch from THORChain to validate signature count matches message count in TSS keysign.

Problem

Without this check, if len(data) > len(n.messages), accessing n.messages[i] could cause an index out of bounds panic.

Changes

Added validation before processing signatures:

if len(data) != len(n.messages) {
    return fmt.Errorf("signature count (%d) != message count (%d)", len(data), len(n.messages))
}

Reference

• THORChain MR: thorchain/thornode!4603 (merged)

Test plan

• Existing TSS keysign tests pass
• No regression in keysign functionality