Exit code for `call` subcommand

Dear Fatih,

thanks a stack for conceiving this excellent tool. We started using it within a CI pipeline to validate the [CrateDB MCP Server](https://github.com/crate/cratedb-mcp) per [mcptools.sh].

While working with `mcpt`, we discovered the program does not provide a non-zero exit code when it calls an unknown MCP tool. That's a canonical example:
```shell
$ mcpt call unknown npx -y @modelcontextprotocol/server-filesystem ~
Error: Unknown tool: unknown

$ echo $?
0
```

Maybe you can do something about it, so the program can be used for validation purposes conveniently, like in our use case?

With kind regards,
Andreas.

[mcptools.sh]: https://github.com/crate/cratedb-mcp/blob/b0bf4dee04a574c57f493abc4ac482ff3feb9d0a/examples/mcptools.sh