azmcp: write missing-subcommand diagnostics to stderr

## Summary

Invoking `@azure/mcp` without a command exits with code 1, but writes the generated root help to **stdout**. Only `Required command was not provided.` is written to **stderr**.

For MCP clients using stdio, stdout is reserved for newline-delimited JSON-RPC messages. A client configured with a bare `npx -y @azure/mcp` command therefore receives hundreds of non-JSON lines and may fail before surfacing the actionable diagnostic.

## Reproduction

Reproduced with the latest published package on July 23, 2026:

```sh
npx -y @azure/mcp@3.0.0-beta.26 >stdout.txt 2>stderr.txt
echo $?
```

Observed:

- Exit code: `1`
- stdout: 297 lines of version and root help text
- stderr: `Required command was not provided.`

The valid invocation, `npx -y @azure/mcp server start`, is not affected.

## Expected behavior

For an invalid invocation caused by a missing command:

- Exit nonzero.
- Write no output to stdout.
- Write the error and generated help/usage text to stderr.

Explicit help requests such as `npx -y @azure/mcp --help` should continue to exit successfully and write help to stdout.

## Acceptance criteria

- A bare invocation exits nonzero with empty stdout.
- The missing-command error and root help are written to stderr.
- Explicit `--help` behavior remains unchanged.
- Valid `server start` stdio framing remains unchanged.
- Automated tests assert exit codes and stdout/stderr routing for the bare invocation and explicit help path.

Start with the @azure/mcp CLI handling for the bare invocation, --help, and server start entry points, then run the automated CLI tests described by the issue. Add coverage for exit codes and stdout/stderr routing: the bare invocation should emit no stdout and put the error and help on stderr, while --help stays on stdout and server start framing remains unchanged.

Written by the indexing model from the issue text.