TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 3: refactor(mcp_client): migrate L451 set_file_slice to Result[T] (Phase 3 site 8)

Added set_file_slice_result(Result[str]) inside the Result Variants region.
Legacy set_file_slice (str) now delegates to set_file_slice_result.

Audit: mcp_client BC count 33 -> 32 (Batch A complete: -8 sites).

src/mcp_client.py

@@ -456,25 +456,16 @@ def get_file_slice(path: str, start_line: int, end_line: int) -> str:
  return "; ".join(e.ui_message() for e in resolved.errors)
 
 def set_file_slice(path: str, start_line: int, end_line: int, new_content: str) -> str:
- """Replace a specific line range in a file with new content."""
- p, err = _resolve_and_check(path)
- if err:
-  return err
- assert p is not None
- if not p.exists():
-  return f"ERROR: file not found: {path}"
- try:
-  lines = p.read_text(encoding="utf-8").splitlines(keepends=True)
-  start_idx = start_line - 1
-  end_idx = end_line
-  if new_content and not new_content.endswith("\n"):
-   new_content += "\n"
-  new_lines = new_content.splitlines(keepends=True) if new_content else []
-  lines[start_idx:end_idx] = new_lines
-  p.write_text("".join(lines), encoding="utf-8")
-  return f"Successfully updated lines {start_line}-{end_line} in {path}"
- except Exception as e:
-  return f"ERROR updating slice in '{path}': {e}"
+ """Replace a specific line range in a file with new content.
+
+ Thin wrapper over set_file_slice_result; the legacy str shape is
+ preserved for backward compatibility, but the try/except Exception
+ lives in the Result variant.
+ """
+ resolved = set_file_slice_result(path, start_line, end_line, new_content)
+ if resolved.ok:
+  return resolved.data
+ return "; ".join(e.ui_message() for e in resolved.errors)
 
 def get_git_diff(path: str, base_rev: str = "HEAD", head_rev: str = "") -> str:
  """