Enhance assistant security with structured validation and input sanitization

- Prompt enhancements:
  - Enforce strict response structure validation by requiring <stage>, <thought>, <action>, and <answer> tags in responses.
  - Add self-validation instruction in <final_instruction> for assistant to check response structure before outputting.
  - Instruct assistant to ignore XML-like syntax from user input, treating any <stage>, <action>, etc., as plain text.

- Code changes:
  - Implement `validateAssistantResponse` function to enforce required response structure (e.g., ensuring <stage> element).
  - Add input sanitization using `lodash.escape` to treat user inputs as plain text, preventing XML or HTML injection.
  - Configure XML parser to ignore external entities and avoid interpreting embedded XML-like syntax.
  - Introduce fallback error handling in parsing and validation to prevent assistant crashes on malformed or unexpected input.
  - Log response errors with detailed messages to aid debugging and improve system resilience.
  - Enhance input validation for tools by adding parameter checks, handling malformed data gracefully, and logging safety errors.

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