fix(claude_sdk,conversations,rotation): context size from the last api call, new-day text by rotation reason

tests/test_claude_sdk_backend.py

@@ -98,7 +98,12 @@ class FakeClient:
         self.interrupted = True
 
     async def receive_response(self):
-        start = {"type": "message_start", "message": {}}
+        start = {
+            "type": "message_start",
+            "message": {
+                "usage": {"input_tokens": 5, "cache_read_input_tokens": 20_000}
+            },
+        }
         yield StreamEvent(uuid="u", session_id="s", event=start)
         for e in _stream(0, "calling "):
             yield e
@@ -119,7 +124,17 @@ class FakeClient:
         yield UserMessage(
             content=[ToolResultBlock(tool_use_id="toolu_1", content="42")]
         )
-        yield StreamEvent(uuid="u", session_id="s", event=start)
+        second = {
+            "type": "message_start",
+            "message": {
+                "usage": {
+                    "input_tokens": 7,
+                    "cache_read_input_tokens": 20_000,
+                    "cache_creation_input_tokens": 3_000,
+                }
+            },
+        }
+        yield StreamEvent(uuid="u", session_id="s", event=second)
         for e in _stream(0, "done"):
             yield e
         yield AssistantMessage(content=[TextBlock(text="done")], model="m")
@@ -194,6 +209,7 @@ async def test_stream_envelope_and_index_rebase(cwd: Path) -> None:
 
     assert capture.session_id == "fresh-session"
     assert capture.usage is not None and capture.usage.cost_usd == 0.01
+    assert capture.usage.context_tokens == 23_007
     assert capture.synthesized_messages == [
         {
             "role": "assistant",