Bug: Default Helm version v3.18.4 is not applied when no version input is provided; v4.0.1 is installed instead

### What happened?

When I use the action without specifying a version
```
- name: Setup Helm
uses: azure/setup-helm@v4
```
It installs `v4.0.1`, which breaks workflows that depend on Helm v3.x.

According to both the documentation and the codebase, the default version should be [v3.18.4](https://github.com/Azure/setup-helm/blob/f838c7624e181517573dd1dd445bd979fa9af13c/src/run.ts#L13). However, this value is not respected.

Please ensure the correct default version is installed (v3.18.4), or update the code/documentation if the default is no longer intended.

### Version

- [x] I am using the latest version

### Runner

ubuntu-latest

### Relevant log output

```
Run azure/setup-helm@v4
with:
version: latest
token: ***
downloadBaseURL: https://get.helm.sh/
env:
xxx: xxxxxxxx # None version related env var
Installing v4.0.1
Downloading 'v4.0.1' from 'https://get.helm.sh/'
/usr/bin/tar xz --warning=no-unknown-keyword --overwrite -C /home/runner/work/_temp/81f0b9e4-b8c6-4c46-a29b-6952ee85cd18 -f /home/runner/work/_temp/9fd0de9e-7488-4808-8d92-d201b505b7b4
Helm tool version 'v4.0.1' has been cached at /opt/hostedtoolcache/helm/4.0.1/x64/linux-amd64/helm
```

Start in src/run.ts at the default version referenced by the issue, then reproduce the provided workflow without a version input and inspect the action log. Done means the no-input path installs Helm v3.18.4, with the documentation and behavior agreeing on that default.

Written by the indexing model from the issue text.