Release v19.13.1

Fixed

• WaitForCompletion() no longer fails with an InvalidOperationException when the append result carries event-type tails. The request's repeated EventTypeTails field defaulted to an array, which protobuf-net cannot add to while decoding, so every non-empty request failed on the server (#4280)
• A WaitForCompletion() timeout response that lists outstanding observers now decodes on the client, for the same reason (#4280)

Compatibility

Only the default values changed; field numbers and property types are unchanged, so the wire schema does not move. Both sides need the fix: an unfixed 19.11.0–19.13.0 server still rejects non-empty tails from a fixed client, and an unfixed client still cannot decode a timeout response that names outstanding observers.

Verification

• Round-trip specifications under Server.Specs/Contracts/ for non-empty tails, omitted tails and non-empty outstanding observers. The two non-empty cases reproduce the exception without the fix.
• All Server specs pass; Release solution build with warnings as errors and formatting pass.
• A search of every protobuf-net contract found no other IEnumerable<T> member with an array default.

Closes #4280