x/gov: leading comma in proposal_messages event attribute

When a proposal is submitted, the `proposal_messages` event attribute is built by prepending a comma to each message type URL: ```go // x/gov/keeper/proposal.go, line 44 msgsStr += fmt.Sprintf(",%s", sdk.MsgTypeURL(msg)) ``` For a proposal with two messages, the emitted value looks like `",/cosmos.bank.v1beta1.MsgSend,/cosmos.staking.v1beta1.MsgDelegate"` instead of `"/cosmos.bank.v1beta1.MsgSend,/cosmos.staking.v1beta1.MsgDelegate"`. The leading comma breaks any downstream consumer that parses this attribute by splitting on commas (produces an empty first element). **Suggested fix:** replace the loop accumulation with `strings.Join` over a `[]string` of type URLs, or trim the leading comma with `msgsStr[1:]` after the loop. ### How I found this Was auditing the gov module's event emission paths and noticed `msgsStr` is initialized empty on line 39, then unconditionally prepended with a comma on every iteration at line 44.