Adds ML Agent and AgentVersion models

Feature context

This MR is part of the AI Agents Incubation Engineering Department project. See this epic for details.

What does this MR do and why?

Solves Add Agent and Agent Version tables and models (#437300 - closed)

Contributes to List Agents (#437299 - closed)

Based on !140859 (closed)

Adds the base models and database tables required for our agent version registry to EE. In the first iteration an agent has a name and many versions. An agent_version has a prompt and a model.

EE specifics

1. Put model code to ee/app/models because this feature is not present in any form in CE right now. Based on the documentation it's also not necessary to add them to the EE namespace.
2. Put test code to ee/spec/ based on https://docs.gitlab.com/ee/development/ee_features.html#code-in-spec

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🚫 backend only

How to set up and validate locally

🚫 database and models only