user_message_uuid stamping behavior for SDK streams clarified

What

• The documented behavior of user_message_uuid changed: it stamps the turn's first top-level assistant message (which may be only a thinking block, or a synthetic API-error message), not simply the first reply frame.
• With --include-partial-messages, the first non-ping stream event is now stamped independently of the completed message, so the same uuid can appear on two frames instead of the partial stream replacing the complete-message stamp.
• The SDK docstring now also clarifies that a turn started by a typed prompt stamps both its first non-ping stream event and, independently, its first complete assistant message (SDKAssistantMessage.user_message_uuid), so both can carry the same uuid.

Why This makes the SDK's documentation match its real streaming behavior, so integrators relying on user_message_uuid to correlate partial and complete messages get accurate expectations instead of being misled by outdated docs.