Core: Add multi-snapshot support to SnapshotChanges

[Shekharrajak]

Ref #15660

• Extend SnapshotChanges from a single snapshot to an arbitrary set of snapshots while preserving the existing single-snapshot API.
• Callers that need changes across an ancestor range (e.g. cherry-pick validation, streaming planners) previously had to loop snapshot-by-snapshot and serialize manifest reads.

Out of Scope

• Streaming accessors / per-snapshot accessors / changesBySnapshot() — belong to Snapshot Changes should have a Streaming Interface #15659.
• Migrating CherryPickOperation and Spark MicroBatchUtils to the new API — separate PR under Snapshot Changes should have a Streaming Interface #15659.
• Removing the deprecated SnapshotUtil.newFiles* methods

[Shekharrajak]

Resolves the zizmor ref-version-mismatch finding that was failing the workflows lint job.