[PR] grpc: populate offset seen from the event when it is omitted [pekko-projection]

["pjfanning (via GitHub)" <gi...@apache.org>]

pjfanning opened a new pull request, #660:
URL: https://github.com/apache/pekko-projection/pull/660

   ### Motivation
   The `seen` entries of the protocol `Offset` are redundant when they only contain the persistence id and sequence number of the enclosing `Event` or `FilteredEvent`. akka/akka-projection#999 made the consumer tolerate an empty `seen` so that producers can omit it.
   
   ### Modification
   - When converting an `Event` or `FilteredEvent` to an `EventEnvelope`, the gRPC consumer fills in `seen` with `persistenceId -> seqNr` of the enclosing event if the protocol offset has no `seen` entries.
   - The conversion moved into an internal helper on the `GrpcReadJournal` companion so it can be tested directly. Akka has this logic in `ProtobufProtocolConversions`, which Pekko doesn't have yet (it arrives with producer push, akka/akka-projection#932).
   - The `event_producer.proto` comment documents that `seen` is optional.
   
   ### Result
   Consumers handle offsets without `seen` correctly.
   
   Rolling upgrades: producers still send `seen`, so this is receiver-side tolerance only. The wire format is unchanged.
   
   ### Tests
   - `sbt "grpc-test/Test/testOnly org.apache.pekko.projection.grpc.consumer.scaladsl.GrpcReadJournalOffsetSpec"`: the empty-`seen` case failed before the fix and passes after; the case with `seen` present passes both before and after.
   - `grpc/mimaReportBinaryIssues`: not applicable, MiMa is disabled for the `grpc` module (new in 2.0.0).
   - Native scalafmt on changed Scala files, `sbt headerCreateAll`, whitespace diff check: clean.
   
   ### References
   Refs akka/akka-projection#999 (https://github.com/akka/akka-projection/pull/999) - source of the ported code. The Akka 1.5.0 change date (2026-10-25) has passed. Upstream had no test; the test here is new.
   


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