Reshape the persistence contracts for groups, redirects and retry batches

[johnsimons]

Renames and reshapes three persistence contracts, plus the RavenDB implementations and API call that follow from them. Split out of the EF Core persister stack so the RavenDB and API changes on their own and don't complicate a release.

No EF Core behaviour changes here: those stores are still stubs, updated only to the new signatures.

Changes

Failure groups. IGroupsDataStore members are named for the status they read:
GetUnresolvedGroupsByClassifier, GetArchivedGroupsByClassifier, GetUnresolvedGroup,
GetArchivedGroup. The two by-id lookups previously differed only in which index they read, which
neither the names nor the signatures showed. GetUnresolvedGroup returns a single view like its
archived twin, so neither controller picks the first item off a list.

QueryFailureGroupViewOnGroupId is deleted, since it ran the same query as GetUnresolvedGroup, and
RetryAllInGroupHandler uses IGroupsDataStore. GetCurrentForwardingBatch moves to the retry
store, next to the NowForwarding document it reads, and returns a ForwardingRetryBatch describing
the operation rather than a RetryBatch whose message list the caller never reads.

Message redirects. IMessageRedirectsDataStore exposes AddRedirect, UpdateRedirect and
RemoveRedirect instead of Save(collection), since every caller changes one redirect at a time.
GetRedirects still returns all of them for the API's duplicate and dependents validation. The
document type moves into the RavenDB project with its stored shape, id and tick based timestamp
unchanged; the shared model keeps the redirect itself, with the lookups as extension methods.
IRetryBatchesManager.GetOrCreateMessageRedirectsCollection duplicated the store's read, so it is
removed and RetryProcessor takes IMessageRedirectsDataStore.

Retry batches. IRetryDocumentDataStore becomes IRetryBatchStore, with CreateBatch,
AssignMessagesToBatch, GetOrphanedBatches and GetAvailableBatchGroups. The four
GetBatchesFor* members returned no batches at all, they enumerate the messages a retry request
covers, so they are now ForEachUnresolvedMessage and friends, minus cutoff, groupTitle and
groupType, which no implementation used.

API behaviour changes

• GET api/recoverability/groups/id/{id} and GET api/archive/groups/id/{id} return 404 for an
  unknown group instead of an empty 204, matching GetErrorByIdController. ServicePulse reads these
  only to label a group and currently fails on the empty 204 anyway.
• The redirect GET endpoints derive their ETag from the payload rather than the document change
  vector. Still deterministic, so conditional requests keep working; clients see one cache miss
  after deploy.