## Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

## The Feature

When registering an A2A agent via `POST /v1/agents`, custom extension fields (e.g., `x-provider-org`, `x-routing-hints`) included in `agent_card_params` are silently stripped and not persisted or returned in the agent card.

Per the [A2A Protocol Specification](https://a2a-protocol.org/latest/specification/), Agent Cards support extensions through custom properties with namespace prefixes to provide additional functionality beyond the core spec. The spec states that extensions can be used to strongly type metadata values for specific use cases.

Currently, LiteLLM's agent registration endpoint only persists known/whitelisted fields from `agent_card_params` (e.g., `name`, `url`, `description`, `skills`, `protocolVersion`, `capabilities`). Any custom `x-` prefixed fields are dropped during storage and therefore absent from the agent card returned via:

- `GET /public/agent_hub` - `GET /a2a/{agent_id}/.well-known/agent-card.json`

## Use Case

In enterprise deployments, organizations need to attach agent-level metadata for discovery and routing purpose…