fix(TASK-044): streaming response.incomplete parity — keep partial output, record usage

Review findings: the streaming arm now mirrors the non-streaming twin
(openai_extract_responses): partial text/tool calls are returned rather
than discarded by a hard error; only an empty truncated response bails.
Billed usage on truncated turns is recorded before either outcome.
Also: CHANGELOG notes the streaming-incomplete surfacing and corrects
the include/strict phrasing (sent on every responses-wire body).

src/client/stream.rs

@@ -215,6 +215,14 @@ impl SseHandler {
         !self.buffer.is_empty() || !self.tool_calls.is_empty() || !self.thinking.is_empty()
     }
 
+    /// Whether visible output (text or tool calls) has been accumulated.
+    /// Unlike [`Self::has_received_content`], thinking blocks do not count:
+    /// a stream truncated before any visible output is still an empty
+    /// response to the caller.
+    pub fn has_received_visible_output(&self) -> bool {
+        !self.buffer.is_empty() || !self.tool_calls.is_empty()
+    }
+
     pub fn abort(&self) -> AbortSignal {
         self.abort_signal.clone()
     }