An empty `data:` frame (or a frame carrying only `event:` / `id:`) is a legal SSE
no-op, but the OpenAI SDK still hands it to `json.loads`, which raises
`JSONDecodeError` with an empty document. The streaming helper translated that
into `ProviderStreamError(provider_stream_non_json_data)`, nothing recognised it
as recoverable, and the main loop retried streaming -- identically -- until the
retry budget ran out: "API call failed after 3 retries: Provider stream returned
non-JSON SSE data". A degraded gateway answers EVERY streaming request that way,
so the retries only repeated the failure while the same request sent
non-streaming succeeded seconds later.
An empty document means no payload, which is a different fact from a malformed
payload: give it its own code, and when it appears before any delta switch the
session to non-streaming (the existing `_disable_streaming` mechanism, already
used for "stream not supported", Bedrock IAM denials and adapter-returned final
responses) so the retry goes out on a channel the degraded gateway can answer.
Non-empty payloads keep today's fatal semantics; failures after deltas keep the
existing stream-drop handling.
Tests: the turn-level recovery test drives the real SDK decoder over a real
httpx response and is red on base (three identical streaming attempts, turn
lost); the boundary test pins the malformed-payload path so a future "ignore bad
frames" change cannot swallow genuine provider errors.
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