Sanitize sync_my_list errors to resolve CodeQL stack-trace exposure alert

[Copilot]

Summary

Code scanning alert #1 (py/stack-trace-exposure) flagged user-visible exception leakage in list sync responses. This change removes raw exception text from API output while preserving response shape and stage-level failure reporting.

• Alert remediation

  • Updated /home/runner/work/Partuno/Partuno/services.py in sync_my_list(...) to replace detail: str(exc) with fixed safe messages in:
    • additions error path
    • updates error path
    • removals error path

• Behavioral impact

  • Clients still receive structured per-stage/per-item errors.
  • Internal exception content is no longer exposed in response payloads.

• Representative change

  # before
  errors.append({"stage": "additions", "detail": str(exc)})
  
  # after
  errors.append({
      "stage": "additions",
      "detail": "Failed to add one or more parts to the list.",
  })

Validation

• python -m pytest -q
• python -m compileall -q .
• Documentation updated where behavior changed
• No credentials, tokens, account identifiers, or live provider data added

Safety and compatibility

• No live provider writes were executed, or the exact authorized test is
  described here.
• MCP tool annotations and REST/OpenAPI compatibility were considered.
• Any provider-specific behavior or limitation is documented.