Deployment: Don't allow parameter files that are only a property bag

See in src/Bicep.LangServer/Deploy/DeploymentParametersHelper.cs:

// We'll work around the above issue by first detecting the format of the file.
// If it's in the format descibed in the docs, we'll extract the parameters value and use that for actual deployment.

We should require the parameters file to be a full JSON file, not just the property bag portion.

Start in src/Bicep.LangServer/Deploy/DeploymentParametersHelper.cs and read the format-detection path described in the issue. Confirm how property-bag input is handled, then make the behavior accept only a full JSON parameters file; done means property-bag-only input is no longer allowed.

Written by the indexing model from the issue text.