fix: astrbot_file_read_tool returns clear error for directory path instead of misleading Permission denied (#9088)

When LLM passes a directory path to astrbot_file_read_tool, the tool previously
returned Error: [Errno 13] Permission denied, misleading the LLM into thinking
it was a permissions issue. The real cause: _probe_local_file() calls open('rb')
on the path, which fails on directories with Errno 13 on Windows. This is caught
by except PermissionError and displayed as-is.

Fix: Add os.path.isdir() check in FileReadTool.call() before any file I/O, at
the earliest safe point after path normalization and permission validation.
Returns a clear message: '<path> is a directory, not a file. Use a file path
instead, or use astrbot_execute_shell to list directory contents.'

Changes:
- astrbot/core/tools/computer_tools/fs.py: add isdir guard
- tests/test_computer_fs_tools.py: add test_file_read_tool_rejects_directory_with_clear_message

astrbot/core/tools/computer_tools/fs.py

@@ -304,6 +304,11 @@ class FileReadTool(FunctionTool):
             )
             if not normalized_path:
                 raise ValueError("`path` must be a non-empty string.")
+            if local_env and os.path.isdir(normalized_path):
+                return (
+                    f"Error: '{normalized_path}' is a directory, not a file. "
+                    "Use a file path instead, or use 'astrbot_execute_shell' to list directory contents."
+                )
             offset, limit = self._validate_read_window(offset, limit)
             sb = await get_booter(
                 context.context.context,

tests/test_computer_fs_tools.py

@@ -620,3 +620,23 @@ async def test_grep_tool_applies_result_limit(
     assert "match-2" in result
     assert "match-3" not in result
     assert "[Truncated to first 2 result groups.]" in result
+
+
+@pytest.mark.asyncio
+async def test_file_read_tool_rejects_directory_with_clear_message(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path,
+):
+    """FileReadTool should return a helpful message when given a directory path."""
+    workspace = _setup_local_fs_tools(monkeypatch, tmp_path)
+    subdir = workspace / "my-directory"
+    subdir.mkdir()
+
+    result = await fs_tools.FileReadTool().call(
+        _make_context(),
+        path="my-directory",
+    )
+
+    assert "is a directory, not a file" in result
+    assert "my-directory" in result
+    assert "'astrbot_execute_shell'" in result