### Check for existing issues

- [x] I searched existing issues. - [x] This is a follow-up/regression report for #20278. PR #20365 fixed the issue, but PR #20446 reverted that fix. The problem is still reproducible in LiteLLM 1.93.0.

### What happened?

LiteLLM's A2A JSON-RPC proxy returns `SendStreamingMessage` responses as newline-delimited JSON:

```http HTTP/1.1 200 OK Content-Type: application/x-ndjson ```

The response body contains multiple complete JSON-RPC objects separated by `\n`:

```text {"jsonrpc":"2.0","id":"...","result":{"task":{...}}} {"jsonrpc":"2.0","id":"...","result":{"statusUpdate":{...}}} {"jsonrpc":"2.0","id":"...","result":{"artifactUpdate":{...}}} {"jsonrpc":"2.0","id":"...","result":{"statusUpdate":{...}}} ```

For the A2A 1.0 JSON-RPC binding, the official `a2a-sdk` client requests `text/event-stream` and expects SSE framing. Because LiteLLM returns NDJSON, the SDK treats the entire non-SSE body as one response and cannot parse multiple JSON-RPC objects as one event.

### Steps to reproduce

1. Register an A2A 1.0 agent that supports streaming. 2. Send this request through LiteLLM:

```bash curl --no-buffer --location 'http://127.0.0.1:4000/a2a/my-agen…