OPE-101: extract token usage in the OpenAI Responses provider

Any model routed through /v1/responses (gpt-5.6+ with tools, i.e. the
default gpt-5.6-sol) reported 0 tokens for every call: both
AssistantTurn constructions omitted usage while every sibling adapter
populated TokenUsage. First consumer to notice was the Auto-Approve
reviewer metering; the 2026-08-13 eval report printed 'Tokens: 0 in /
0 out' across ~40 live calls.

_usage_from maps the Responses shape like the Chat Completions adapter:
fresh input = input_tokens - cached_tokens (input_tokens is INCLUSIVE
of the cached share), cached share -> cache_read, output_tokens as-is
(reasoning already included). Defensive reads: compat servers may omit
input_tokens_details; a missing usage object stays None, never a fake
zero. Wired in _parse_response, which both complete() and the stream's
terminal response.completed event flow through - one extraction, both
paths.

Tests: cache split, partial/missing degradation, stream terminal event,
stream-without-terminal stays None. Verified live against gpt-5.6-sol:
call 1 'in=1820 cache_read=0', call 2 'in=3 cache_read=1817' - the
0-token era ends and the cache split reports correctly.

tests/test_openai_responses.py

@@ -273,6 +273,39 @@ def test_complete_text_turn_and_request_shape():
     assert fake.kwargs["reasoning"] == {"summary": "auto"}
 
 
+def test_complete_extracts_usage_with_cache_split():
+    # OPE-101: the Responses API reports `input_tokens` INCLUSIVE of the cached share;
+    # normalized like every other adapter — fresh input = input − cached, cache_read
+    # carries the cached share. Before this, the field was dropped entirely and every
+    # Responses-routed model metered as 0 tokens.
+    resp = _response([_message_item("hello")])
+    resp.usage = SimpleNamespace(
+        input_tokens=1500,
+        output_tokens=80,
+        input_tokens_details=SimpleNamespace(cached_tokens=1400),
+    )
+    provider = OpenAIResponsesProvider(client=_FakeClient(response=resp))
+    turn = provider.complete(model="m", messages=[{"role": "user", "content": "hi"}])
+    assert turn.usage is not None
+    assert (turn.usage.input, turn.usage.output, turn.usage.cache_read) == (100, 80, 1400)
+
+
+def test_complete_usage_degrades_on_partial_or_missing_fields():
+    # Compat/older servers may omit `input_tokens_details` or the whole usage object —
+    # never a crash, and absence stays None (not a fake zero-usage).
+    resp = _response([_message_item("x")])
+    resp.usage = SimpleNamespace(input_tokens=500, output_tokens=20)  # no details
+    provider = OpenAIResponsesProvider(client=_FakeClient(response=resp))
+    turn = provider.complete(model="m", messages=[{"role": "user", "content": "hi"}])
+    assert (turn.usage.input, turn.usage.output, turn.usage.cache_read) == (500, 20, 0)
+
+    bare = _response([_message_item("y")])  # SimpleNamespace without a usage attr at all
+    turn2 = OpenAIResponsesProvider(client=_FakeClient(response=bare)).complete(
+        model="m", messages=[{"role": "user", "content": "hi"}]
+    )
+    assert turn2.usage is None
+
+
 def test_complete_parses_function_calls_with_call_ids():
     fake = _FakeClient(
         response=_response(
@@ -498,6 +531,28 @@ def test_stream_without_terminal_event_keeps_accumulated_text():
         provider.stream(model="m", messages=[{"role": "user", "content": "x"}])
     )[-1].turn
     assert turn.text == "partial" and turn.finish_reason is None
+    assert turn.usage is None  # nothing terminal arrived — no usage to invent
+
+
+def test_stream_terminal_event_carries_usage():
+    # OPE-101 streaming path: usage rides the terminal `response.completed` event's full
+    # response object, which the stream parses whole — same extraction as complete().
+    final = _response([_message_item("done")])
+    final.usage = SimpleNamespace(
+        input_tokens=1430,
+        output_tokens=65,
+        input_tokens_details=SimpleNamespace(cached_tokens=1408),
+    )
+    events = [
+        SimpleNamespace(type="response.output_text.delta", delta="done"),
+        SimpleNamespace(type="response.completed", response=final),
+    ]
+    provider = OpenAIResponsesProvider(client=_FakeClient(events=events))
+    turn = list(
+        provider.stream(model="m", messages=[{"role": "user", "content": "x"}])
+    )[-1].turn
+    assert turn.usage is not None
+    assert (turn.usage.input, turn.usage.output, turn.usage.cache_read) == (22, 65, 1408)
 
 
 def test_stream_requests_stream_flag():