Add implementation of returnable last_seq

[veronika-alraheem]

PR summary

Add implementation of returnable last_seq for safe checkpoint management.

Fixes: i1286

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

The ChangesFollower API streams changes as individual ChangesResultItem objects but provides no mechanism for users to determine a safe sequence ID to use as a checkpoint. For heavily filtered feeds where pages return empty results, users have no way to advance their checkpoint even though the feed has progressed internally.

What is the new behavior?

A new public method getLastSeqNewerThan(lastPersistedSeqId) is added to ChangesFollower. After fully processing a ChangesResultItem, users can call this method with the item's seq value to obtain the most recent sequence ID that is safe to persist as a checkpoint.

The method advances past any empty pages (e.g. from heavily filtered feeds) that the follower has seen since the given sequence ID, returning the furthest safe checkpoint available at that point.

Does this PR introduce a breaking change?

• Yes
• No