fix(tui): preserve usage telemetry fields

tests/webui/test_session_context.py

@@ -59,7 +59,12 @@ def test_session_context_tolerates_untrusted_summary_metadata() -> None:
 
 
 def test_session_context_sanitizes_usage_metadata() -> None:
-    usage = LLMUsage.reported(input_tokens=120, output_tokens=8, total_tokens=175)
+    usage = LLMUsage.reported(
+        input_tokens=120,
+        output_tokens=8,
+        total_tokens=175,
+        cache_read_tokens=48,
+    ).with_timing(generation_ms=400, ttft_ms=75)
     session = Session(
         key="websocket:context",
         metadata={"_last_usage": usage.to_dict()},
@@ -67,4 +72,16 @@ def test_session_context_sanitizes_usage_metadata() -> None:
 
     payload = session_context_payload(session)
 
-    assert payload["last_usage"] == usage.to_dict()
+    assert payload["last_usage"] == {
+        "prompt_tokens": 120,
+        "completion_tokens": 8,
+        "total_tokens": 175,
+        "context_tokens": 120,
+        "cached_tokens": 48,
+        "request_count": 1,
+        "estimated_tokens": 0,
+        "generation_ms": 400,
+        "measured_completion_tokens": 8,
+        "ttft_ms": 75,
+        "timed_requests": 1,
+    }