## Failure
The encoder fuzz host relied on NUnit or corpus-tool setup to assign
`FuzzableCode.MessageContext`. Direct afl-fuzz/libFuzzer startup could
therefore
reach encoder targets without an initialized OPC UA message context.
## Fix
- Initialize one process-lifetime `ServiceMessageContext` with null
logging.
- Make the context immutable and remove test/tool reassignment.
- Exercise the same context from regression tests and the standalone
fuzz host.
## Reference
- OPC 10000-6 encoding validation.
- Split from #4021.
## Tests
- `dotnet test
fuzzing\Opc.Ua.Encoders.Fuzz.Tests\Opc.Ua.Encoders.Fuzz.Tests.csproj -c
Release -f net10.0 --nologo` (4,569 passed)
- `dotnet run --project
fuzzing\Opc.Ua.Encoders.Fuzz\Opc.Ua.Encoders.Fuzz.csproj -c Release --`
(standalone smoke passed)
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Marc Schier