Migrate explain_vulnerability prompt

What does this MR do and why?

Related to Migrate Duo Chat Tools: ExplainVulnerability (#475046 - closed)

In this MR, we:

• Add prompt_migration_explain_vulnerability FF
• Migrate the ExplainVulnerability tool to use the AIGW
• Use UseAiGatewayAgentPrompt concern with a FF

How to set up and validate locally

• Setup runners and the ExplainVulnerability context
• Run AI Gateway with Anthropic debug: ANTHROPIC_LOG=debug poetry run ai_gateway
• Checkout migrate-explain-vulnerability-tool in the AIGW
• Enable prompt_migration_explain_vulnerability feature flag on Rails (::Feature.enable(:prompt_migration_explain_vulnerability))
• Perform /vulnerability_explain slash command in Duo Chat, eg.. /vulnerability_explain #include <stdio.h> int main(int argc, char *argv[]) { char buf[8]; memcpy(&buf, "123456789"); printf("hello, world!"); }
• Verify that /v1/prompts/chat//vulnerability_explain endpoint is called in the AIGW
• Verify that the correct prompt is sent to Anthropic

I recorded a quick demo of the test results since setting up the environment would be very time-consuming.

Screen_Recording_2024-08-29_at_10.04.56