feat(chat): stream model thinking over SSE and show it in a collapsible block

tests/unit/test_sse_events.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 import json
 
 from app.api.chat import sse_event
-from app.schemas import ChatErrorEvent
+from app.schemas import ChatErrorEvent, ChatThinkingEvent
 
 
 def _payload(frame: str) -> dict:
@@ -61,3 +61,18 @@ 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
+
+
+def test_thinking_frame_serializes_exactly() -> None:
+    """Phase 17 (PLAN §4 extension): the ``thinking`` frame is exactly
+    ``{type: "thinking", text: str}`` — the sibling shape of ``delta``
+    the client's readSSE handler will branch on."""
+    frame = sse_event(ChatThinkingEvent(text="Step 1: check the docs…").model_dump())
+    assert frame == 'data: {"type": "thinking", "text": "Step 1: check the docs…"}\n\n'
+    assert _payload(frame) == {"type": "thinking", "text": "Step 1: check the docs…"}
+
+
+def test_thinking_event_shape_is_type_and_text_only() -> None:
+    dumped = ChatThinkingEvent(text="hmm").model_dump()
+    assert set(dumped.keys()) == {"type", "text"}
+    assert dumped["type"] == "thinking"  # default — call sites never spell it out