fix: read工具单行超长时内容被完全丢弃

单行超过 MAX_CHARS(128K) 时，截断循环 end_idx=0 导致 lines[..0] 为空，
整行内容丢失。改为当首行就超长时使用 take_prefix_chars 字符级截断。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tools/file_read.rs

@@ -3,6 +3,7 @@ use std::path::Path;
 use async_trait::async_trait;
 use serde_json::json;
 
+use crate::text::take_prefix_chars;
 use crate::tools::traits::{Tool, ToolResult};
 use crate::tools::extract_u64;
 
@@ -187,8 +188,13 @@ impl Tool for FileReadTool {
                         }
                         end_idx = i + 1;
                     }
-                    result = lines[..end_idx].join("\n");
-                    let truncated_amount = original_len - result.len();
+                    if end_idx == 0 && !lines.is_empty() {
+                        // First line alone exceeds MAX_CHARS — take its prefix
+                        result = take_prefix_chars(&lines[0], MAX_CHARS.saturating_sub(100));
+                    } else {
+                        result = lines[..end_idx].join("\n");
+                    }
+                    let truncated_amount = original_len.saturating_sub(result.len());
                     result.push_str(&format!(
                         "\n\n... ({} chars truncated) ...",
                         truncated_amount
@@ -312,4 +318,28 @@ mod tests {
         assert!(!result.success);
         assert!(result.error.unwrap().contains("Not a file"));
     }
+
+    #[tokio::test]
+    async fn test_read_single_long_line() {
+        let mut file = NamedTempFile::new().unwrap();
+        // Write a single line longer than MAX_CHARS
+        let long_line = "A".repeat(150_000);
+        file.write_all(long_line.as_bytes()).unwrap();
+
+        let tool = FileReadTool::new();
+        let result = tool
+            .execute(json!({ "path": file.path().to_str().unwrap() }))
+            .await
+            .unwrap();
+
+        assert!(result.success);
+        // Should contain the line number prefix and the beginning of the content
+        assert!(result.output.starts_with("1| AAAA"));
+        // Should contain truncation notice since content exceeds MAX_CHARS
+        assert!(result.output.contains("chars truncated"));
+        // Should contain end-of-file notice (1 line total)
+        assert!(result.output.contains("End of file — 1 lines total"));
+        // Should NOT be empty content — the fix ensures the prefix is preserved
+        assert!(result.output.len() > 100);
+    }
 }