Migrate WriteTests tool

What does this MR do and why?

Related to Migrate Duo Chat Tools: WriteTests (#475049 - closed)

In this MR, we:

• Add prompt_migration_write_tests FF
• Migrate the WriteTests 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 475049-migrate-write-tests in the AIGW
• Enable prompt_migration_write_tests feature flag on Rails (::Feature.enable(:prompt_migration_write_tests))
• Perform /tests slash command in Duo Chat, eg.. /tests def hello_world(): print("Hello, World!")
• Verify that /v1/prompts/chat/write_tests endpoint is called in the AIGW
• Verify that the correct prompt is sent to Anthropic