Add Duo Agent User Environment context support

What does this MR do and why?

References

This MR adds another "Duo Context" category. Currently you can include open tabs, dependencies, git information, issues, MRs as reference to your Duo (Agentic) Chat.

This MR adds agent_user_environment category. This context will contain any user-environment related info: OS, shell, etc., so that chat can provide more targeted scripts, commands.

Check this MR to see how agent_user_environment context (User Shell) is included in the Agentic Chat request

How to set up and validate locally

1. enable the feature flag echo "Feature.enable(:duo_include_context_agent_user_environment)" | rails c

2. run the following GraphQL query in the explorer http://gdk.test:3000/-/graphql-explorer

   query getDuoAvailableFeatures {
     currentUser {
       duoChatAvailableFeatures
     }
   }

3. and see the response, important is that include_agent_user_environment_context is present

{
  "data": {
    "currentUser": {
      "duoChatAvailableFeatures": [
        "include_file_context",
        "include_snippet_context",
        "include_user_rule_context",
        "include_repository_context",
        "include_agent_user_environment_context"
      ]
    }
  },
}

MR acceptance checklist

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Related issue

#548507 (closed)