Migrate Backend to Duo Chat

During the VE development sync, a new work flow was proposed using /explain_vulnerability. Our current backend implementation may need to be updated or even migrated over.

Workflow:

1. Keep the current vulnerability explanation section
2. User clicks on "explain vulnerability" button
3. Which opens the Duo chat drawer
4. It triggers the /vulnerability_explain slash command
5. Displays AI response

Things that may need updating or migration:

• Prompt Message: explain_vulnerability.rb
• GraphQL Mutation: AiExplainVulnerabilityInput
• ...possibly others

Other Notes:

1. make sure you also implement the /explain_vulnerability command fully, so that users don't have to click the button but can also just type this command. Read more here
2. ~~Also supporting explain_vulnerability_with_source_code, ~~read more here
   1. @sming-gitlab will verify with PM on this and confirm actual text, link.
3. The default is "send with code". Ignore secret and preflight checks.
4. We're going to proceed with Anthropic per latest discussion with model validation team.
   1. But keep in mind: "We need the flexibility to change depending on the result of the model validation"

Implementation Plan

...