After streaming retries are exhausted on transient errors, fall back to
non-streaming instead of propagating the error. Also fall back for any
other pre-delivery stream error (not just 'streaming not supported').
Added user-facing message when streaming is not supported by a model/
provider, directing users to set display.streaming: false in config.yaml
to avoid the fallback delay.
Cherry-picked from PR #3008 by kshitijk4poor. Added UX message for
streaming-not-supported detection.
Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>
Previously the fallback only triggered on specific error keywords like
'streaming is not supported'. Many third-party providers have partial
or broken streaming — rejecting stream=True, crashing on stream_options,
dropping connections mid-stream, returning malformed chunks, etc.
Now: any exception during the streaming API call triggers an automatic
fallback to the standard non-streaming request path. The error is logged
at INFO level for diagnostics but never surfaces to the user. If the
fallback also fails, THAT error propagates normally.
This ensures streaming is additive — it improves UX when it works but
never breaks providers that don't support it.
Tests: 2 new (any-error fallback, double-failure propagation), 15 total.
