Malformed SSE chunk drops already-streamed text from history #62

Bug

`GenericProvider._stream` (`pycodeloop/providers/generic.py:397`) does `chunk = json.loads(payload)` with no `try/except`. If an OpenAI-compatible endpoint sends a truncated/corrupt `data: ` line mid-stream (network blip, proxy cutting the body, etc.), that `json.loads` raises.

Trace

By that point, `on_delta(...)` (lines 413-424) has already fired for every prior chunk — those `chat/textDelta` notifications already reached the client and rendered in the UI. But the exception propagates up through `Agent._complete`; since it's not `TimeoutError`/`ConnectionError` it isn't retried, the turn fails, and `session.add_assistant(...)` is never called. The text the user already saw on screen never enters the persisted session — reloading the session shows the conversation without that reply.

Fix

Wrap the malformed-chunk case so streaming degrades gracefully instead of losing already-shown text — e.g. catch the JSON decode error, stop consuming the stream, and still persist whatever text/tool-calls were accumulated so far (similar to how a clean `stop_reason` is handled), rather than letting the exception discard everything.