fix(tui): surface concrete run errors instead of a bare error header

StreamEvent::Error is encoded by the agent loop as an assistant message with
stop_reason=error and error_message, but the UI only rendered the header and
the (empty) content, hiding the actual message (e.g. a provider HTTP 400
detail). Now:

- UiBlock::Error renders error_message in red, for both committed and live
  assistant messages
- on job Done, the error is also surfaced into the status bar and a note,
  even when the job itself reported no error (it was encoded as a message)
- regression test: an error message renders its concrete text

crates/focus-tui/src/app.rs

@@ -438,8 +438,22 @@ impl App {
 
                 self.persist_entries(&to_persist);
                 self.transcript = messages;
-                if let Some(err) = error {
-                    self.status = format!("⚠ {}", err);
+                // 错误优先取 job 报告的 error；若错误被编码成消息（如 provider 的
+                // HTTP 400 → StreamEvent::Error → assistant error 消息），
+                // 从最后一条 assistant 消息的 error_message 提取。
+                // Prefer the job's reported error; when the error is encoded as
+                // a message (provider HTTP 400 → StreamEvent::Error → an
+                // assistant error message), extract it from the last assistant.
+                let surfaced = error.or_else(|| {
+                    self.transcript.iter().rev().find_map(|m| match m {
+                        Message::Assistant(a) if a.stop_reason == StopReason::Error => {
+                            a.error_message.clone()
+                        }
+                        _ => None,
+                    })
+                });
+                if let Some(err) = surfaced {
+                    self.status = format!("⚠ {}", crate::format::truncate(&err, 80));
                     self.push_note(format!("运行出错：{}", err));
                 } else {
                     self.status = "✓ 完成".into();

crates/focus-tui/src/state.rs

@@ -77,6 +77,9 @@ pub enum UiBlock {
         is_error: bool,
         expanded: bool,
     },
+    /// 运行错误（assistant 消息的 error_message，红色显示）。
+    /// A run error (an assistant message's error_message, shown in red).
+    Error { id: BlockId, text: String },
     /// 工具执行过程（运行中 / 完成，含耗时与输出）。
     /// A tool execution (running / done, with duration and output).
     ToolExec {
@@ -382,6 +385,14 @@ pub fn build_blocks(
                         expanded,
                     );
                 }
+                // 具体错误文本（如 provider 的 HTTP 400 详情）。
+                // The concrete error text (e.g. a provider HTTP 400 detail).
+                if let Some(err) = &a.error_message {
+                    out.push(UiBlock::Error {
+                        id: BlockId(format!("m{}-err", mi)),
+                        text: err.clone(),
+                    });
+                }
             }
             Message::ToolResult(t) => {
                 out.push(UiBlock::ToolResult {
@@ -414,6 +425,12 @@ pub fn build_blocks(
                     expanded,
                 );
             }
+            if let Some(err) = &partial.error_message {
+                out.push(UiBlock::Error {
+                    id: BlockId("live-err".into()),
+                    text: err.clone(),
+                });
+            }
         }
         for (ei, exec) in rs.tool_execs.iter().enumerate() {
             out.push(UiBlock::ToolExec {
@@ -485,7 +502,8 @@ impl UiBlock {
             | UiBlock::Thinking { id, .. }
             | UiBlock::ToolCall { id, .. }
             | UiBlock::ToolResult { id, .. }
-            | UiBlock::ToolExec { id, .. } => Some(id),
+            | UiBlock::ToolExec { id, .. }
+            | UiBlock::Error { id, .. } => Some(id),
             UiBlock::Note { .. } => None,
         }
     }

crates/focus-tui/src/ui.rs

@@ -271,6 +271,14 @@ fn block_lines(block: &UiBlock, width: usize) -> Vec<Line<'static>> {
                 )));
             }
         }
+        UiBlock::Error { text, .. } => {
+            for line in wrap_text(text, width) {
+                out.push(Line::from(Span::styled(
+                    line,
+                    Style::new().fg(Color::Red).bold(),
+                )));
+            }
+        }
         UiBlock::ToolCall {
             name,
             summary,

crates/focus-tui/tests/state_tests.rs

@@ -279,3 +279,26 @@ fn working_phase_detection() {
     });
     assert_eq!(rs.working_phase(), "streaming…");
 }
+
+/// 错误消息必须渲染出具体的错误文本（而不是只有消息头）。
+/// An error message must render its concrete error text (not just a header).
+#[test]
+fn error_message_renders_as_error_block() {
+    let transcript = vec![Message::Assistant(AssistantMessage {
+        content: Vec::new(),
+        model: "m".into(),
+        usage: Usage::default(),
+        stop_reason: StopReason::Error,
+        error_message: Some("http error 400: bad request detail".into()),
+        timestamp: 0,
+    })];
+    let blocks = build_blocks(&transcript, &HashMap::new(), None, &HashSet::new());
+    let err = blocks
+        .iter()
+        .find_map(|b| match b {
+            UiBlock::Error { text, .. } => Some(text.clone()),
+            _ => None,
+        })
+        .expect("error block");
+    assert_eq!(err, "http error 400: bad request detail");
+}