feat(rust): introduce snapshot api

[majin1102]

Implement #5885

[github-actions]

Code Review: feat(rust): introduce snapshot api

Summary

This PR adds a new Snapshot and Snapshots API for accessing dataset version history with lazy loading and timestamp-based filtering capabilities.

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P0 Issues (Must Fix)

1. Timestamp Semantics Issue - last_modified vs created_at

The last_modified field from object store metadata represents when the manifest file was last modified on disk, not when the version was created. For newly committed manifests, the code uses Utc::now() which is the commit time, but for existing manifests read from storage, it uses the file's last_modified time. These can differ if:

• The file was copied/moved
• The file system was restored from backup
• Object store replication delays

This inconsistency could cause within_timestamp() and earlier_than_timestamp() to return unexpected results.

Recommendation: Consider using the manifest's internal timestamp() field (already available via Manifest::timestamp()) for reliable version timestamps, or clearly document this limitation in the API.

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P1 Issues (Should Fix)

1. Inefficient Repeated Listing in Filtering Methods

Methods like within(), earlier_than(), and later_than() each call list().await? separately, which does a full manifest listing every time. If a user calls multiple filtering methods, they'll hit the object store multiple times unnecessarily.

// Each of these does a separate listing
let a = snapshots.within(1, 4).await?;
let b = snapshots.earlier_than(3, 2).await?;

Recommendation: Consider caching the manifest list within the Snapshots struct, or document that users should call list() once and filter locally for multiple queries.

2. later_than Semantics Mismatch with Documentation

The method later_than(version, limit) is documented as "Get snapshots later than the given version" but the implementation uses >= version:

.filter(|s| s.version_number() >= version)  // includes the version itself

This is inconsistent with earlier_than which correctly uses < version. The same issue exists in later_than_timestamp.

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Minor Suggestions (Non-blocking)

• The Snapshot and Snapshots structs could benefit from documentation comments explaining their purpose and usage patterns
• Consider making ManifestLocation visibility consistent (it's now exposed through the public Snapshot::manifest_location field)

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Testing

Good test coverage overall. Tests appropriately use in-memory storage and cover edge cases.