[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

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