fix(fixp): preserve outbound journal on suspended reap

[pedrosakuma]

Closes #594

Summary

• stop SuspendedTimeout from purging the durable outbound journal while still reaping the session slot and removing the resumability snapshot
• reseed fresh sessions from any preserved journal history before they emit new outbound sequence numbers
• add regression coverage for suspended-timeout preservation and the related takeover timing path

Validation

• dotnet test tests/B3.Exchange.Gateway.Tests/B3.Exchange.Gateway.Tests.csproj --no-restore -c Release --filter "FullyQualifiedName~EntryPointListenerReaperTests|FullyQualifiedName~FixpSessionPersistenceWiringTests|FullyQualifiedName~FixpSessionMassActionTakeoverTests|FullyQualifiedName~Persistence.FileFixpOutboundJournalTests"
• dotnet build SbeB3Exchange.slnx --no-restore -c Release
• dotnet test SbeB3Exchange.slnx --no-build -c Release
• dotnet build SbeB3Exchange.slnx --no-restore -c Debug
• dotnet test SbeB3Exchange.slnx --no-build -c Debug (reran once after an unrelated transient FixpSessionRetransmitTests EOF flake; rerun passed clean)
• dotnet format SbeB3Exchange.slnx --verify-no-changes --no-restore --severity warn