[Feat]: Unique identifier for an agent in Agent Card

### Is your feature request related to a problem? Please describe.

Currently Agent Card does not have a unique identifier attribute for an agent. `agent_card.name` is not guaranteed unique and is prone to branding changes.

### Describe the solution you'd like

- Add a new`id` attribute to the agent card which must be a randomly generated UUID.
- This `id` must remain same for all versions of an agent.
- Mark the `id` as optional for backward compatibility.
- Clients can reliably use `id` as the identifier if set in the agent card.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look for the agent card definition in the codebase, likely in a schema or model file. Understand how UUIDs are generated and stored in the project. Check existing tests for agent cards to see how to add and validate the new optional id field. Ensure backward compatibility by making the field optional and updating any serialization/deserialization logic.

Scritto dal modello di indicizzazione a partire dal testo della issue.