com.atproto.sync.getRecord should prove non-existence of records

[uniphil]

Currently, calling sync.getRecord on a bridgy repo returns 400 error="InvalidRequest" for records that don't exist.

eg: https://atproto.brid.gy/xrpc/com.atproto.sync.getRecord?did=did:plc:ic4mplmy2blzwvurli4htcim&collection=fake.collection.hey&rkey=22234

It should return 200 OK with a CAR slice proving the absence of the requested key -- a slice containing the would-be-adjacent keys.

From the spec:

Get data blocks needed to prove the existence or non-existence of record in the current version of repo.

eg: https://porcini.us-east.host.bsky.network/xrpc/com.atproto.sync.getRecord?did=did:plc:hdhoaan3xa3jiuq4fg4mefid&collection=fake.collection.hey&rkey=22234

[uniphil]

(for lightrail this means it always falls back on full getRepo fetches instead of describeRepo to enumerate the collections list when backfilling, since it tries an expected-non-existent sync.getRecord request first to establish the current repo rev from the Commit object, and repo size estimate from the root node layer, etc).

[snarfed]

Yes! Great point, absolutely true. Thank you for filling!