#3164 Docs: document DatastoreSyncService.fetchContent in the datastore provider reference

What changed

swamp-club#3159 adds an optional method to the DatastoreSyncService contract that extension datastores implement:

fetchContent(relPath, options) returns the bytes of one file as the remote holds it, or null when the remote has no such file. It writes nothing to the local cache.

Page to update

content/manual/reference/extensions/datastore-provider.md. Its sync service method table lists hydrateFile but has no row for fetchContent.

Suggested content

A table row: fetchContent, signature (relPath: string, options?: DatastoreSyncOptions) => Promise of Uint8Array or null, not required, reads one remote file without touching the local cache.

A short paragraph with the contract:

• Returns the file's bytes, or null when the remote has no such file. Any other failure rejects, so an unreachable remote is never read as a deleted file.
• Writes nothing locally: no cache file, dirty state or pull watermark changes, and a differing local file is left alone.
• relPath is cache-relative and forward-slash-normalised. With a namespace it already starts with the namespace, so the implementation must not add it again. With no namespace in options, the path is read from the datastore root.
• A relPath that is absolute or has a dot-dot segment is rejected.
• The returned bytes are not kept on the service instance.
• Core treats the method's presence as the capability; there is no SyncCapabilities flag.

Also mention that the round-trip conformance suite in the testing package has two cases for it, fetch-content and fetch-content-namespace, and that the options paragraph on that page should list fetchContent among the methods that take DatastoreSyncOptions.