test(om): run Event Hubs recovery coverage in the regular suite

[egil]

Summary

Run the enriched Event Hubs token factory and cache recovery cases in the normal test suite now that all OM packages require Orleans 10.4. Remove the superseded 10.3.1 consumer matrix and document that consumers must upgrade their Orleans host packages together.

Shared #285 prerequisite → Orleans 10.4.0 across stable packages
Preview Journaling → 10.4.0-alpha.1
Regular Event Hubs suite → domain, metadata, serialization and recovery coverage

This draft is stacked on egil/om-orleans-compatibility / #286. The shared prerequisite already contains the provider-domain override, consistent package pins and core adaptations; this PR owns only the test relocation, obsolete matrix removal and Event Hubs documentation updates. There is no mixed compile/runtime reference strategy or older-runtime support matrix.

Persisted checkpoint aliases, field IDs and JSON discriminators remain unchanged. Historical exact V1 recovery is retained as a persisted-data requirement, not an older-runtime support promise.

Refs #282; coordinated acceptance remains owned by #285. Do not close #282 or merge this draft independently of the shared upgrade.

Evidence

• Before: six factory/cache cases ran only through the standalone version-matrix script.
  After: all 44 Event Hubs tests, including those cases, run through the regular solution test command and CI suite.
• Fixed snapshot 5209ccf05d5228e2ec1608fe86d9fd1eac3a6e97, shared base 7b104beba5864b519f925e69110e1989e1db6fa7: solution restore succeeds; Release build has zero warnings/errors; all 976 solution tests pass; solution Release pack produces all four packages. These results use actual committed 10.4 pins without temporary overlays or warning exceptions.
• Retained coverage: symmetric equality/order/hash, sequence-before-index ordering, persisted standard/enriched provider/full-stream checkpoint lookup, JSON and Orleans serialization, custom/null-container delivery, inherited per-event factory metadata, real pooled-cache recovery for standard/enriched/historical exact V1 checkpoints, incompatible public token domains and genuine eviction misses.
• Repeat: dotnet test Egil.Orleans.Messaging/Egil.Orleans.Messaging.slnx -c Release.
• Independent review at the fixed snapshot: no new Standards or Spec findings. Actual StreamManager resume coverage is being integrated by build(om): upgrade all OM packages consistently to Orleans 10.4 #285; live Event Hubs broker/pulling-agent handshake remains an explicitly unverified infrastructure path. In-process cache tests do not establish that path.

Merge Danger

Door: two-way

Blast Radius: EventHubs

Requires the coordinated Orleans 10.4 upgrade in #286. Runtime minimum changes do not permit losing persisted checkpoint data. No merge or release is requested.