[Bug]: Clarify streaming errors in REST transport binding

[yarolegovich]

What happened?

Currently there's no information on how to return errors for streaming requests in REST transport binding.
For JSONRPC which also uses SSE for streaming we return a data line with jsonrpc error struct in it.

data: {"jsonrpc": "2.0", "id": 1, "result": { /* Task | Message | TaskArtifactUpdateEvent | TaskStatusUpdateEvent */ }}

data: {"jsonrpc": "2.0", "id": 1, "error": { /*  details */ }}

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[darrelmiller]

What kind of error would you expect to get during a stream? What would a client do with that error? The Task can go into failed state if a error has prevented the task from completing and then the error information would be included in the TaskState.Message.

I

[yarolegovich]

What kind of error would you expect to get during a stream?

Mostly dependency failures like:

• Required agent dependency (eg. ADK session store) temporarily unhealthy.
• Task store connection issue.

What would a client do with that error?

Good question. Clients might use the error to understand that their request is retryable after a jittered delay during which the unhealthy component might recover.