Add snippets for LanguageModelFailurePlugin to snippets.json

[garrytrinder]

Add new code snippets to src/snippets.json for the LanguageModelFailurePlugin, as documented at https://learn.microsoft.com/microsoft-cloud/dev/dev-proxy/technical-reference/languagemodelfailureplugin.

Requirements:

• Add a snippet for a LanguageModelFailurePlugin instance.
• Add a snippet for the LanguageModelFailurePlugin configuration section.
• Follow the existing format used for other plugins in src/snippets.json (see @garrytrinder/dev-proxy-toolkit/files/src/snippets.json for reference).
• Include an example plugin instance and configuration section in the new snippets.

Snippet Examples:

Plugin instance:

{
  "name": "LanguageModelFailurePlugin",
  "enabled": true,
  "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
  "configSection": "languageModelFailurePlugin"
}

Plugin config section:

"languageModelFailurePlugin": {
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v1.0.0/languagemodelfailureplugin.schema.json",
  "rate": 20,
  "failures": [
    "rate-limit-reached",
    "quota-exceeded"
  ]
}

Acceptance Criteria:

• New snippets appear in src/snippets.json for LanguageModelFailurePlugin, matching the established style and structure.
• Example values are provided for plugin instance and configuration section, consistent with official documentation.
• Snippet descriptions are clear and follow the convention used for other plugin entries.

Please see the official documentation and the existing file for reference.