Standalone mode --with-tendermint false does not work with GRPC and transaction simulation

[p-offtermatt]

Summary of Bug

When starting a node with out-of-process Comet, i.e. --with-tendermint false,
it is not possible to simulate transactions e.g. to determine transaction costs with
--gas auto. I am running with out-of-process Comet, and transactions work fine, except when I add --gas auto,
which gives an error:
Error: rpc error: code = Unknown desc = unknown query path: unknown request

I believe the error lies in the startup, where the TxService is registered when starting with in-process Comet

cosmos-sdk/server/start.go

Line 342 in ec4d9b7

app.RegisterTxService(clientCtx)

, but that functionality is missing when starting standalone Comet here

cosmos-sdk/server/start.go

Line 209 in ec4d9b7

func startStandAlone(ctx *Context, appCreator types.AppCreator) error {

Version

ec4d9b7

Steps to Reproduce

Run simd with --with-tendermint false and out-of-process Comet, then run a transaction with --gas auto.

[p-offtermatt]

Hey, I played with this today and created this PR #18109 for how I managed to do it on v0.47.

I also have a PR targetting main #18110, but since my testscript isn't up to compatibility with Cosmos SDK main, I didn't have a good way to test this.

With #18109, I can query the grpc server fine for everything I tried. Also, tx simulation worked for me - it fixes the use cases I need, but I assume it doesn't work for everything since I don't give it an actual client context.
Maybe the PRs are still useful as a base-line though.