invalid `applies_to` keys can be set in front matter

[shainaraskas]

Describe the bug

I passed the following key:

applies_to:
  deployment:
      serverless: all
      hosted: all

hosted and serverless is not a valid subkey of deployment, so the little tag didn't appear in my page. however, the PR passed CI with no problems

Expected behavior

invalid keys throw CI errors

I realize that we want to allow custom keys, but in the other well-defined key/subkey areas we should do some validation

Steps to reproduce

applies_to:
  deployment:
      serverless: all
      hosted: all

Tooling

• docs-builder
• migration tooling
• I'm not sure

[bmorelli25]

Might require "hint" phase before erroring.

[theletterf]

Currently, docs-builder emits a warning if an unknown key is used. From ApplicableTo.cs:

docs-builder/src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs

Lines 145 to 147 in b133c9d

	var unknownKeys = keys.Except(KnownKeys).Except(oldStyleKeys).ToList();
	if (unknownKeys.Count > 0)
	warnings.Add($"Applies block does not support the following keys: {string.Join(", ", unknownKeys)}");

Warnings don't block the CI. If you want to throw an error, then the code should be:

		var unknownKeys = keys.Except(KnownKeys).Except(oldStyleKeys).ToList();
		if (unknownKeys.Count > 0)
		{
			// Change from warning to error by throwing an exception
			throw new YamlException($"Invalid applies_to configuration: Unknown keys found: {string.Join(", ", unknownKeys)}");
		}

Or perhaps using EmitError, @Mpdreamz ? Trying to understand the codebase little by little...