Add comprehensive list indexing error handling and tests

[lwgray]

Description:
This PR adds robust error handling for list indexing operations in the Think language interpreter, along with comprehensive test coverage to verify the behavior.

Changes

• Add validation for list indices before access:
  • Type checking (must be integer or whole number)
  • Bounds checking (both positive and negative indices)
  • Validation of nested list access
• Improve error messages to provide clearer feedback
• Move error handling to be proactive rather than reactive

New Tests

Added two test suites to verify error handling:

def test_list_indexing_errors(self, interpreter, parser, capture_output):
    """Test error cases for list indexing."""
    # Test index out of bounds (positive)
    # Test index out of bounds (negative) 
    # Test non-integer index
    # Test invalid type for index

def test_nested_list_indexing_errors(self, interpreter, parser, capture_output):
    """Test error cases for nested list indexing."""
    # Test accessing index of non-list
    # Test out of bounds on nested list

Testing

• All existing tests pass
• New test coverage verifies error cases:
  • Out of bounds indices
  • Non-integer indices
  • Invalid type indices
  • Nested indexing errors

Example Error Messages

Invalid index/key: 5 is out of range for list of length 3
Invalid index/key: List indices must be integers, got str
Cannot index into type: int

Breaking Changes

None. This PR only improves error handling without changing existing behavior.