The SDK URL was incorrectly changed to a2aprojects/a2a-python (404 broken link).
Revert to the correct a2aproject/a2a-python which is the official A2A Python SDK org.
Fixes blocker from PR review.
Corrects the Example Streaming Event in docs/reference/a2a.md to use
the correct JSON-RPC 2.0 notification method name `task/statusUpdate`
instead of the incorrect `message/stream`.
`message/stream` is the client→server REQUEST method that initiates
streaming. The actual SSE data payloads are JSON-RPC 2.0 notifications
(no `id` field) with method names `task/statusUpdate` or
`task/artifactUpdate`, as implemented in SseEventFormatter.
Closes#2519