Migrate FixCode tool

What does this MR do and why?

Migrate Duo Chat Tools: FixCode (#475047)

In this MR, we:

• Add prompt_migration_fix_code FF
• Migrate the FixCode tool to use the AIGW
• Use UseAiGatewayAgentPrompt concern with a FF

How to set up and validate locally

• Run AI Gateway with Anthropic debug: ANTHROPIC_LOG=debug poetry run ai_gateway
• Checkout id-fix-code-prompts in the AIGW
• Enable prompt_migration_fix_code feature flag on Rails (::Feature.enable(:prompt_migration_fix_code))
• Perform /fix slash command in Duo Chat, eg.. /fix def hello_world(): print("Hello, World!")
• Verify that /v1/prompts/chat/fix_code endpoint is called in the AIGW
• Verify that the correct prompt is sent to Anthropic