fix(transit/message): yield legit JSON null payload in ReceiveAllAsync (fixes #220)

[Kiryuumaru]

Problem (fixes #220)

MessageTransit<TSend, TReceive>.ReceiveAllAsync overloaded null as a sentinel for two distinct conditions:

1. End-of-stream — ReceiveAsync returns default after the length-prefix read returns 0 bytes
2. A peer-sent JSON null payload — 4-byte length prefix + 6E 75 6C 6C; deserializes to null for nullable TReceive

The receive loop checked if (_receiveEof || message is null) yield break;. The value-type half of this bug was previously fixed (#177) by adding the explicit _receiveEof flag, but the secondary message is null clause was kept and still terminates the stream on the first legitimate JSON null message. Every subsequent peer message is silently lost — no exception, no event, no log.

Example (pre-fix): peer sends "hello", null, "world". Consumer's await foreach over ReceiveAllAsync() yields only "hello" and exits. "world" is permanently dropped.

Fix

Drop the secondary message is null terminator. _receiveEof is the sole signal — and it is reliably set inside ReceiveAsync whenever the length-prefix or payload read returns 0 bytes. The yield is message! because a legit JSON null payload is a valid yielded value for nullable TReceive.

if (_receiveEof)
    yield break;

yield return message!;

No public-API change.

Regression Test

Added in tests/NetConduit.Transit.Message.UnitTests/MessageTransitTests.cs under #region ReceiveAllAsync:

• MessageTransit_ReceiveAllAsync_YieldsLegitimateJsonNullPayload_AndContinues — MessageTransit<string?, string?> sends "hello", null, "world"; asserts the receiver yields all three in order before EOF.

Pre-fix verification (stashed the prod change, ran the new test):

MessageTransit_ReceiveAllAsync_YieldsLegitimateJsonNullPayload_AndContinues [FAIL]
  Assert.Equal() Failure: Collections differ
                                 ↓ (pos 1)
  Expected: ["hello", null, "world"]
  Actual:   ["hello"]

Post-fix: passes.

Existing tests MessageTransit_ReceiveAllAsync_StopsOnChannelClose and ..._WhenReceiveTypeIsValueType (covering EOF for reference and value-type TReceive respectively, #177) continue to pass — _receiveEof correctly drives termination for both cases.

Verification

• dotnet build -c Debug — 0 warnings, 0 errors across net8.0 / net9.0 / net10.0
• dotnet test -c Debug --no-build — 635 passed / 1 known-flake (PublicApiTests.OnChannelOpened_DoesNotFire_BeforeRemoteAccept, passes in isolation) / 1 pre-existing skip (MemoryLeak_SubMuxChaos_MemoryStaysBounded)
• Targeted re-run of the flake in isolation: 1 passed