BotMaker collection plus same defect class in Last()

[Alb4don]

This is a follow-up to #58, which reported three unvalidated collection helpers in the BotMaker runtime. I re checked current main while verifying them I found a fourth instance of the same defect class in Last(), which that issue did not cover.

• Affected:

     botmaker/src/java/com/twitter/botmaker/function/collection/First.java, Last.java, FirstN.java, Slice.java
  

• New finding: Last()

      // Last.java (~L49)
        return list.get(list.size() - 1);
  

• On an empty collection this resolves to get(-1) and throws IndexOutOfBoundsException.

• Same root cause as First(), one function later in the file, not mentioned in BotMaker collection functions crash on edge cases: missing validation for empty lists and invalid indices #58.

• FunctionNode1.toExtractor() wraps the exception in a FunctionFailure so that the exception does not halt the worker; however, the enclosing rule does not complete, meaning within the scarecrow flow that the intended label is never applied.

• Based on the code, I could not determine whether the calling component treats this as fail-open (label ignored for that event) or fail-closed (event reprocessed), as the scarecrow dispatch layer and part of the provided rule set are not included in this repository.

• This is the specific point I would ask the maintainers to clarify, as it determines whether the practical impact is a robustness issue or a vector for bypassing labeling.

• I compiled the four files from main against minimal stubs of the internal dependencies (com.twitter.util.Future, the annotation processor types, etc.) and drove each crash condition directly:

• The harness downloads the four files from raw.githubusercontent.com, compiles them unmodified, and exits non zero when any trigger fails to crash

• Suggested fix

Validate at the function boundary and surface a typed FunctionFailure with a descriptive message, rather than letting the JDK throw unadorned:

      if (list == null || list.isEmpty()) {
          throw new FunctionFailure(this, context.getStackFrames(),
              new IllegalArgumentException("First() called on empty list"));
      }
      
      long count = Math.max(0L, n);
      return list.subList(0, (int) Math.min(count, list.size()));
      
      
      int max = input instanceof String ? ((String) input).length() : ((List<?>) input).size();
      if (beginIndex < 0 || endIndex < 0 || beginIndex > endIndex || endIndex > max) {
          throw new IllegalArgumentException(
              String.format("Invalid slice indices: begin=%d, end=%d, length=%d", beginIndex, endIndex, max));
      }