fix(sse_client): address Volva code-vs-contract drift (issue #1)

Volva's code-spec review (thread 01KS4CP6ZZ1F) surfaced four code-vs-
contract drift findings on the TDD-passing implementation. All four
addressed here; no contract amendments required.

1. _iter_events fell off the end of aiter_sse() normally on clean EOF
   before any Done/Error/Cancelled. Per INV-001 the iterator MUST NOT
   raise StopAsyncIteration before a terminal event unless the HTTP
   connection drops, in which case it raises SseConnectionDropped.
   Clean EOF before terminal is the same semantic — the stream ended
   without delivering its contracted invariant. Fix: track terminal_seen
   inside _iter_events; after the async-for completes, if not seen,
   raise SseConnectionDropped(last_seen_sse_id=...). Two new tests:
   test_clean_eof_before_terminal (one text then EOF) and
   test_zero_event_eof (empty stream — last_seen_sse_id is None).

2. SseConnectFailed and CancelFailed both store .body without
   truncation; ERROR_ROUTING specifies resp.read()[:1024]. Fix
   truncates in each exception's __init__ before storing. New test
   test_connect_failed_body_truncated (503 + 5000-byte body → 1024)
   and test_cancel_failed_truncates_body (same shape on cancel).

3. _parse_sse_id PRE-001 specifies `assert isinstance(raw, str)`.
   Previous code called raw.split(":") directly, which raises an
   incidental AttributeError on non-str inputs — not the contracted
   precondition path. Fix adds the assert. New test
   test_non_string_input covers int and None.

4. Cancel ERROR_ROUTING said httpx.HTTPStatusError other status →
   CancelFailed, but no test exercised the branch. test_cancel_failed_
   truncates_body covers this (above) — single test double-covers
   findings 2 and 4.

43 tests GREEN (42 sse_client + boundary smoke); ruff clean.

Meta-note from Volva: TDD caught the main happy/adversarial SSE shape,
resume header/body, turn-id flip, and cancel races. The remaining
misses were "negative space" cases (clean premature EOF, exception
payload truncation, untested generic cancel branch). Calibration
evidence that cross-model review pulls weight on what same-model
TDD's hypothesis-space doesn't probe.

tests/test_sse_client.py

@@ -114,6 +114,13 @@ class TestParseSseId:
         assert raw not in str(exc_info.value), "full 5000-char input must not appear in message"
         assert raw[:64] in str(exc_info.value), "first 64 chars must appear in message"
 
+    def test_non_string_input(self) -> None:
+        """PRE-001 hard: raw is a string -- isinstance check before parse."""
+        with pytest.raises(AssertionError):
+            _parse_sse_id(42)  # type: ignore[arg-type]
+        with pytest.raises(AssertionError):
+            _parse_sse_id(None)  # type: ignore[arg-type]
+
 
 class TestStreamTurn:
     @respx.mock
@@ -367,6 +374,54 @@ class TestStreamTurn:
         assert len(yielded) == 1
         assert exc_info.value.last_seen_sse_id == SseId(42, 1)
 
+    @respx.mock
+    async def test_clean_eof_before_terminal(self) -> None:
+        """INV-001: clean EOF without Done/Error/Cancelled -> SseConnectionDropped."""
+        from ratatoskr.sse_client import SseConnectionDropped
+
+        # One text event, then stream ends cleanly (no terminal).
+        stream = _sse_chunk("42:1", {"type": "text", "content": "x"})
+        respx.post("https://w.example/sessions/s1/messages").mock(
+            return_value=httpx.Response(
+                200, headers={"content-type": "text/event-stream"}, content=stream
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            yielded: list[Text] = []
+            with pytest.raises(SseConnectionDropped) as exc_info:
+                async for e in stream_turn(client, "s1", "hi"):
+                    yielded.append(e)  # type: ignore[arg-type]
+        assert len(yielded) == 1
+        assert exc_info.value.last_seen_sse_id == SseId(42, 1)
+
+    @respx.mock
+    async def test_zero_event_eof(self) -> None:
+        """INV-001: empty stream (no events at all) -> SseConnectionDropped(None)."""
+        from ratatoskr.sse_client import SseConnectionDropped
+
+        respx.post("https://w.example/sessions/s1/messages").mock(
+            return_value=httpx.Response(
+                200, headers={"content-type": "text/event-stream"}, content=b""
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SseConnectionDropped) as exc_info:
+                _ = [e async for e in stream_turn(client, "s1", "hi")]
+        assert exc_info.value.last_seen_sse_id is None
+
+    @respx.mock
+    async def test_connect_failed_body_truncated(self) -> None:
+        """ERROR_ROUTING: SseConnectFailed.body is truncated to <= 1024 bytes."""
+        big_body = b"x" * 5000
+        respx.post("https://w.example/sessions/s1/messages").mock(
+            return_value=httpx.Response(503, content=big_body)
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SseConnectFailed) as exc_info:
+                _ = [e async for e in stream_turn(client, "s1", "hi")]
+        assert len(exc_info.value.body) <= 1024
+        assert exc_info.value.body == big_body[:1024]
+
     @respx.mock
     async def test_no_text_aggregation(self) -> None:
         """no_text_aggregation: consumer yields each text event separately; no concat."""
@@ -601,6 +656,22 @@ class TestCancelTurn:
                 await cancel_turn(client, "s1", 42)
         assert exc_info.value.turn_id == 42
 
+    @respx.mock
+    async def test_cancel_failed_truncates_body(self) -> None:
+        """ERROR_ROUTING: 5xx -> CancelFailed; body truncated to <= 1024 bytes."""
+        from ratatoskr.sse_client import CancelFailed
+
+        big_body = b"x" * 5000
+        respx.post("https://w.example/sessions/s1/turns/42/cancel").mock(
+            return_value=httpx.Response(503, content=big_body)
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(CancelFailed) as exc_info:
+                await cancel_turn(client, "s1", 42)
+        assert exc_info.value.status == 503
+        assert len(exc_info.value.body) <= 1024
+        assert exc_info.value.body == big_body[:1024]
+
     @respx.mock
     async def test_cancel_idempotent(self) -> None:
         """cancel_idempotent: first 200 -> happy; second 409 -> CancelAlreadyCompleted."""