feat(ui): explicit chat state machine — typing indicator, streaming progress, timeout and error recovery

tests/unit/test_sse_events.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 import json
 
 from app.api.chat import sse_event
+from app.schemas import ChatErrorEvent
 
 
 def _payload(frame: str) -> dict:
@@ -46,3 +47,17 @@ def test_multi_line_text_stays_one_frame() -> None:
     frame = sse_event({"type": "delta", "text": "line1\nline2\n\n"})
     assert frame.count("\n\n") == 1  # only the frame terminator
     assert _payload(frame)["text"] == "line1\nline2\n\n"
+
+
+def test_error_event_model_serializes_exact_frame() -> None:
+    """The ``ChatErrorEvent`` model is the wire shape of every server-side
+    failure the UI's state machine (phase 06) must recover from."""
+    frame = sse_event(ChatErrorEvent(detail="boom").model_dump())
+    assert frame == 'data: {"type": "error", "detail": "boom"}\n\n'
+    assert _payload(frame) == {"type": "error", "detail": "boom"}
+
+
+def test_error_event_shape_is_type_and_detail_only() -> None:
+    dumped = ChatErrorEvent(detail="The chat model dropped the connection").model_dump()
+    assert set(dumped.keys()) == {"type", "detail"}
+    assert dumped["type"] == "error"  # default — call sites never spell it out