- Add messageId to start chunk (required by useChat)
- Remove undocumented tool-output-error wire type, use
tool-output-available for all tool results
- Pass structured tool output through instead of JSON-stringifying
Tests cover extractUserText with v5+ parts format, v4 content string,
last-user-message extraction, simple text/prompt fields, null cases, and
preference ordering. Stream listener tests verify text event translation
and the writableEnded guard.
