Add AGENTS.md, add proxy tests for Cognito-IDP

[whummer]

Add proxy tests for the Cognito-IDP service.

🤖 Disclosure: Please note that the code in the PR was fully generated by an AI agent (Gemini), using the following prompt:

Follow the instructions in AGENTS.md and add a new proxy test for the AWS cognito-idp service, inside the ./tests/ directory

An AGENTS.md file has been added to the repo, to steer the actions of the agent and provide guidance on how to add and run tests.