Integrate "Explain Vulnerability" with DuoChat

What does this MR do and why?

The code changes introduce a new feature that allows users to explain vulnerabilities and get mitigation advice using AI. It includes a button to trigger the AI assistant, which opens a chat window with a pre-filled prompt. The feature is powered by a GraphQL mutation that communicates with the AI service. Additionally, a deprecated version of the component is kept for backward compatibility.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Enable the explain_vulnerability_tool feature flag (echo "Feature.enable(:explain_vulnerability_tool)" | rails c)
2. Visit a SAST vulnerability
3. Click on "Explain Vulnerability" and verify that DuoChat opens, and the /explain_vulnerability command is sent

Related to #466080 (closed)