Make AI Agents form available in AI Assistants

As per my understanding these are the required changes to be done. Pointing this for maintainers confirmation.
Redesign the AI Assistant Form Layout:

• Modify the existing AI Assistant configuration form to resemble the AI Agents form. This means consolidating the agent selection and creation process into a single interface.
• Replace the current "Use agent as assistant" dropdown with a more integrated field that allows both selecting an existing agent and creating a new one on the fly.

Create Agent on Save:

• When the user saves the AI Assistant configuration, implement logic to check if a "New agent" option is selected.
• If so, programmatically create a new agent using the provided details (e.g., the administrative description and selected tools).
• Automatically link this newly created agent to the AI Assistant by updating the assistant's configuration to reference the agent's ID.

Integrate Agent Selection Field:

• Add a dynamic dropdown or autocomplete field labeled "AI Agent" (as suggested in your description) that lists existing agents.
• If the user selects "New agent," display an inline form to input agent details (e.g., name, description, and tool preferences) before saving.
• Upon saving, the system should create the agent, assign it the selected tools, and connect it to the AI Assistant in one action.

This is not the greatest code, since it has to have backwards compability for current assistants and we can't force AI Agents to exists in core until 2.0. Alot will be refactored before 2.0, so this is a solution for better assistants for 1.1.0

Since it has a circular dependency, this will be removed in Drupal 2.0.0. It will still support backwards compability for 1.x branch, so you can create assistants without agents.

See ✨ Move AI Agents into AI core module Active for breaking change in 2.0.0.

This is getting merged. I will do testing against Drupal CMS, just to see its 100% backward compatible, otherwise there might be follow up issue.