chore(dkg): remove summary's transcripts for remote subnets (1/4)

[pierugo-dfinity]

Since the work performed to speedup the latency of remote DKG requests, their results are delivered as part of data blocks instead of summaries. The transcripts_for_remote_subnets field in summaries is thus now unused. This PR begins the migration to remove it entirely.

We cannot simply replace the field with a BackwardsCompatible<Vec<_>, _> (i.e. replace the Vec<_> with an Option<Vec<_>>) because the field is a repeated, and an empty vector is not differentiable from an absent one. In contrast, Some(vec![]) and None have different hashes. For that reason, we need to add a new sentinel field to the protobuf definition that will be set to true to explicitely indicate that the field is absent. The entire rollout can be reviewed by looking at the stacked PRs (except the last one) and is described below.

• V0 (current) <-> V1 (after this PR):
  • The goal of V1 is to keep the exact same representation (including hash) of V0's summaries while still understanding the sentinel and compute the same hash as V2 for V2's summaries.
  • Upgrade V0 -> V1: A V0's summary doesn't set the sentinel so the field deserializes as Some(vec![]) and the hash of an empty vector is appropriately computed.
  • Rollback V1 -> V0: a V1's summary doesn't set the sentinel just yet and V0 correctly deserializes. It also computes the same hash as V1.
• V1 <-> V2 (#11221):
  • The goal of V2 is to actually stop hashing an empty vector, and instead just ignore it.
  • Upgrade V1 -> V2: V2 still understands Some(vec![]) so will compute V1's summary's hash identically. For its own summaries, it starts to fill the sentinel (because the transcripts are None).
  • Rollback V2 -> V1: V1 understands the sentinel and thus ignores the transcripts while computing the hash.
• V2 <-> V3 (#11223):
  • The goal of V3 is remove the field entirely.
  • Upgrade V2 -> V3: V3 doesn't know about the transcripts anymore (the field is removed). V2 created a summary with None transcripts meaning their hash was not included, good.
  • Rollback V3 -> V2: V3 unconditionally sets the sentinel to true, meaning V2 will interpret the transcripts as None and not try to hash them.
• V3 <-> V4 (#11224):
  • The goal of V4 is to remove the introduced sentinel field.
  • Upgrade V3 -> V4: V4 will ignore the sentinel field set by V3.
  • Rollback V4 -> V3: V3 will deserialize a summary with an unset sentinel, but it does not read it anyways.

[Copilot]

Pull request overview

This PR is the first step of a staged migration to remove transcripts_for_remote_subnets from DkgSummary without changing summary hashing across replica versions, by introducing a protobuf presence marker and switching the Rust field to a backwards-compatible representation.

Changes:

• Wrap DkgSummary::transcripts_for_remote_subnets in BackwardsCompatible<…> and update protobuf (de)serialization to preserve hash behavior across versions using a new sentinel marker.
• Add try_from_proto_with to BackwardsCompatible to support conversions that aren’t expressed via TryFrom.
• Adjust consensus handling/tests to treat the summary’s remote transcripts as optional (and effectively absent going forward).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rs/types/types/src/consensus/dkg.rs	Switch summary remote transcripts to BackwardsCompatible and add marker-aware protobuf conversions.
rs/types/types/src/backwards_compatibility.rs	Document field-removal lifecycle and add try_from_proto_with helper.
rs/protobuf/src/gen/types/types.v1.rs	Regenerate protobuf bindings to include the new optional marker field.
rs/protobuf/def/types/v1/dkg.proto	Add transcripts_for_remote_subnets_removed sentinel (tag 16) and bump next id.
rs/consensus/src/consensus/batch_delivery.rs	Only generate remote-DKG responses from summaries when transcripts are present.
rs/consensus/dkg/src/test_utils.rs	Stop reading remote transcripts from summary blocks (return empty).
rs/consensus/dkg/src/lib.rs	Update test expectation for the BackwardsCompatible summary field.

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