Enhance default interaction function and improve token metrics handling

[nmaguiar]

This pull request improves the way token usage statistics are calculated and logged throughout the mini-a.js codebase. The main change is the introduction of a new helper method for robustly computing total token counts, which is now used consistently across summary, response, and fallback handling logic. Additionally, the interaction logging function is made more flexible, and a method to retrieve the instance ID is added.

Token statistics calculation and logging:

• Added a new helper method _getTotalTokens to compute total token usage from stats objects, handling cases where total_tokens may be missing and falling back to summing prompt_tokens and completion_tokens. This method is now used in all places where token metrics are updated, improving consistency and robustness.
• Updated all metric logging in _startInternal and _runChatbotMode to use _getTotalTokens, ensuring that token counts are always computed accurately for normal, low-cost, and fallback model responses. [1] [2] [3] [4] [5] [6]

Interaction logging improvements:

• Enhanced defaultInteractionFn to accept an optional callback function (cFn) for custom logging behavior, with a default that preserves previous functionality. [1] [2]

API enhancement:

• Added a new method getId to retrieve the unique ID of a MiniA instance.