fix: Optimize creation of changesets from collections

[keelerm84]

Previously, we worked to cache the changset -> collections process as it
is used in many different places. There are multiple places where a
changeset is created from a collection. With this change, we take
advantage of this knowledge, and pre-cache the collection that created
the changeset, preventing us from doing the conversion at all.

---

Note

Introduce NewChangeSetFromCollections and refactor data sources/tests to build and pass collections directly, avoiding redundant conversions.

• Subsystems:
  • Add NewChangeSetFromCollections to construct a ChangeSet directly from []ldstoretypes.Collection, caching collections and generating changes via collectionsToChanges.
• Data Sources:
  • FDv1 polling (internal/datasourcev2/fdv1_polling_data_source.go): use NewChangeSetFromCollections with existing collection data instead of ChangeSetBuilder.
  • File data source (ldfiledatav2/file_data_source_impl.go): build flag/segment collections directly (via insertDataIntoCollection) and return a ChangeSet via NewChangeSetFromCollections.
• Test Helpers:
  • ldtestdatav2: construct full-transfer collections for flags/segments and use NewChangeSetFromCollections to produce the ChangeSet.

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