Snapshot Support

[zhiyuanliang-ms]

Why this PR?

Support the snapshot APIs.

Snapshot storage

• Metadata: All snapshot metadata persisted as newline-delimited JSON records in .aace/snapshot.ndjson. New snapshot uses Append; update/remove rewrites whole file via temp file + atomic Replace.
• Content: Each snapshot’s captured key-values serialized as NDJSON into its own file (snapshotId.ndjson) under .aace/snapshots folder.

Snapshot management

• Provision: On snapshot Create, the emulator immediately gathers matching key-values, writes the content NDJSON file, computes media info (size, hash, etag), and then appends the snapshot metadata in one flow. A successful PUT request returns a snapshot already in Ready state (no intermediate Provisioning state). This is different from the async provision behavior of the real App Config service.
• Clean up (lazy deletion): Purge only runs once, triggered inside SnapshotProvider.EnsureInit after loading metadata cache. (Different from KeyValueProvider's periodic cleanup logic)

[RichardChen820]

The snapshot name passed from the controller parameter is escaped or not? Why the name using in create method is not escaped, but it is escaped here?

[zhiyuanliang-ms]

The snapshot name passed from the controller parameter is escaped or not?

No. It is literal.

Why the name using in create method is not escaped, but it is escaped here?

This is actually a good catch. First thing, store unescaped name is the bahavior of real app config service.

But using escaped name for search is actually a bug in emulator. This bug also exists in the kv controller.

For example, if we put a key: my*key, it will be stored literally. But when we get it through /kv/my*key, it will return 404 instead of 200.

[zhiyuanliang-ms]

Fixed in #99

[RichardChen820]

SnapshotProvider also runs as a backgroud hosted service, is this intentional？Maybe isolate the hosted servivce from the provider is better

[zhiyuanliang-ms]

This is intentional. I refered to KeyValueProvider. The StartAsync will warm the cache and purge the archieved snapshot