Fix Javadoc and parameter checks in FrameAggregator

This change refines the Javadoc parameter descriptions and exception
documentation in FrameAggregator to avoid private field references and clarify
preconditions, and strengthens constructor validation.

- Corrects parameter descriptions for sequenceIndex across operational
  methods and replaces the private field link in @throws annotations.
- Replaces "non-positive" with "negative" in sequenceIndex documentation
  since index 0 is a valid sequence index.
- Validates that numSequences is strictly positive upon construction and
  adds corresponding unit tests.

PiperOrigin-RevId: 939823423