Initialize encoder fuzz ServiceMessageContext (OPC 10000-6)

[marcschier]

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 Harden security and reliability across the OPC UA stack #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)

[Copilot]

Pull request overview

Note

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This PR ensures the encoder fuzz host always has an initialized OPC UA ServiceMessageContext even when started directly (e.g., afl-fuzz/libFuzzer), by moving initialization into the fuzz target code and removing test/tool reassignment.

Changes:

• Initialize a single process-lifetime ServiceMessageContext in FuzzableCode using a null-logging telemetry context.
• Remove prior test/tool setup that reassigned FuzzableCode.MessageContext.
• Add a regression test asserting MessageContext is available without per-test setup.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
fuzzing/Opc.Ua.Encoders.Fuzz/FuzzableCode.cs	Adds default, process-lifetime ServiceMessageContext initialization using null logging telemetry.
fuzzing/Opc.Ua.Encoders.Fuzz.Tools/Encoders.Testcases.cs	Stops reassigning the global message context; keeps telemetry parameter for signature compatibility.
fuzzing/Opc.Ua.Encoders.Fuzz.Tests/Issue3546RoundTripTests.cs	Removes NUnit setup that previously created/reassigned the message context.
fuzzing/Opc.Ua.Encoders.Fuzz.Tests/EncoderTests.cs	Replaces per-test setup override with a regression test for context initialization.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.56%. Comparing base (12f770b) to head (2821b12).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4035      +/-   ##
==========================================
- Coverage   73.86%   73.56%   -0.31%     
==========================================
  Files        1345     1345              
  Lines      180038   180081      +43     
  Branches    31678    31683       +5     
==========================================
- Hits       132993   132472     -521     
- Misses      36290    36897     +607     
+ Partials    10755    10712      -43     

see 46 files with indirect coverage changes

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[marcschier]

/azp run

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