[Feat]: Register `application/agent-card+json` as an IANA Media Type

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

Currently, there is no registered media type for the Agent Card specification. Without an official media type, interoperability across different discovery mechanisms and implementations is harder, and downstream tooling cannot reliably recognize or validate `agent-card.json` resources.

### Describe the solution you'd like

Register `application/agent-card+json` in the IANA Media Types registry, using a stable URL to the Agent Card specification.

### Describe alternatives you've considered

- Continue using the generic `application/json` media type (lacks specificity, makes automated handling harder).
- Define custom or vendor-specific media types under `application/vnd.*+json` (less ideal for long-term interoperability).

### Additional context

https://github.com/a2aproject/A2A/pull/642#issuecomment-3114354120

### Code of Conduct

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

The issue is about registering a media type with IANA. Start by reading the IANA media type registration process documentation and the existing Agent Card specification. Look at the linked pull request comment for context. Determine the steps to submit the registration, including drafting the required template and coordinating with the project maintainers. 'Done' means the media type is officially registered.

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