Set ContinuationPoint to null when browse is finished (port of #4057)

[romanett]

Ports the ContinuationPoint fix from master commit a568a7e (PR #4057) back to master378.

What changed

MasterNodeManager.Browse/BrowseNext now explicitly set BrowseResult.ContinuationPoint = default (i.e. null) when no continuation point is present, instead of leaving it as an empty array. This aligns server behavior with the existing 1.5.xxx contract.

Three call sites in Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs were updated:

• initial browse result
• bad validation result in BrowseNext
• good result in BrowseNext

Tests

Added Tests/Opc.Ua.Server.Tests/MasterNodeManagerBrowseTests.cs, porting the relevant test coverage from #4057 and adapting it to the master378 server test API (BrowseResultCollection, byte[] continuation point, OperationContext ctor):

• BrowseAsyncCompletedBrowseReturnsNullContinuationPointAsync — a completed browse returns Good and a null continuation point.
• BrowseAsyncUnknownNodeReturnsNullContinuationPointAsync — an unknown node returns BadNodeIdUnknown and a null continuation point.

Not ported

The original commit also touched files that do not exist on master378:

• samples/PumpDeviceIntegrationServer/Dockerfile (unrelated diagnostics-socket change; sample not present)
• The v2.0-era test files (BrowseContinuationPointTests.cs, BrowseTests.cs, MasterNodeManagerDeterministicTests.cs) that were added in the v2.0 refactor and don't exist on master378.

Validation

• dotnet build Libraries/Opc.Ua.Server (net8.0, Release): succeeded, 0 warnings.
• dotnet test Tests/Opc.Ua.Server.Tests --filter MasterNodeManagerBrowseTests (net8.0): 2 passed.

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.20%. Comparing base (db256c6) to head (a3461a8).

Files with missing lines	Patch %	Lines
...ies/Opc.Ua.Server/NodeManager/MasterNodeManager.cs	66.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master378    #4266   +/-   ##
==========================================
  Coverage      60.19%   60.20%           
==========================================
  Files            378      378           
  Lines          79067    79079   +12     
  Branches       13838    13838           
==========================================
+ Hits           47598    47607    +9     
- Misses         27047    27056    +9     
+ Partials        4422     4416    -6     

Files with missing lines	Coverage Δ
...ies/Opc.Ua.Server/NodeManager/MasterNodeManager.cs	66.43% <66.66%> (+<0.01%)	⬆️

... and 8 files with indirect coverage changes

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