Add Streaming Events documentation (follow-up to #205)

[vrtnis]

Following up on #205, which fixed agent_updated_stream_event to correctly emit the new agent after a handoff.

The current Streaming guide explains how to consume streams, but doesn’t document what the actual event types look like. There’s no reference for:

• raw_model_stream_event
• run_item_stream_event
• agent_updated_stream_event

The Python SDK already documents such events in detail (with field definitions and usage examples). It would be great to bring the JS docs up to parity. This has also been requested in #202 which #205 resolved.

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Proposal

Update streaming.mdx with:

• A brief overview of the event system
• Descriptions of each event type and their payload structure
• JSON examples for common scenarios like handoffs and tool use
• A link from the main Streaming guide

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I'm opening this issue ahead of the docs PR to provide visibility and context for reviewers.

Will follow up shortly with a PR to implement this.