Add structure-backed OptionSet DataType support in ComplexTypeSystem

[marcschier]

Concrete sub-types of the abstract OptionSet DataType (i=12755, e.g. AccessRights, CarExtras, OptionSetBase) were silently skipped by ComplexTypeSystem.LoadBaseStructureDataTypesAsync when only an EnumDefinition or OptionSetValues property was available. This change registers them with a new runtime class Opc.Ua.Encoders.OptionSet that reuses the generated Opc.Ua.OptionSet wire format (Value/ValidBits ByteStrings) and self-registers via IEncodeableType.

Key changes:

• New Stack/Opc.Ua/Types/OptionSet.cs with EnumDefinition-driven bit accessors (by name and index), GetSetFieldNames helper, and a fixed ByteLength derived from the highest declared bit per OPC UA Part 3 paragraph 8.40 / 3.2.8. SetBit throws ArgumentOutOfRangeException outside that range and always materializes Value/ValidBits to the fixed length.

• New AddOptionSetType entry point on IComplexTypeBuilder (source-breaking addition for custom implementations); DefaultComplexTypeBuilder implements it. The Reflection.Emit ComplexTypeBuilder throws NotSupportedException.

• ComplexTypeSystem detects OptionSet descendants via a HasSubtype walk and resolves the EnumDefinition from DataTypeDefinition or the OptionSetValues property fallback before the old skip path.

• Removed stale V1.04 OptionSet limitation remark from ComplexTypeSystem.

• 10 unit tests in Tests/Opc.Ua.Client.Tests/ComplexTypes/DefaultOptionSetTests.cs covering factory registration, bit accessors, binary round-trip via ExtensionObject, hand-crafted Part 6 wire format, Clone/CreateInstance semantics, and the ByteLength invariant.

• Live-verified against opc.tcp://DESKTOP-BGFJS91:48010 (UA C++ SDK demo): AccessRights/CarExtras/OptionSetBase now appear in the registered custom-type list; no skip-log entries; LoadAsync(throwOnError:true) returns true.

• Docs/ComplexTypes.md Known Limitations reworded; Docs/MigrationGuide.md gains an OptionSet DataType support subsection.

UInteger-backed OptionSet DataTypes remain intentionally opaque (carried as their unsigned integer in a Variant).

Proposed changes

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[codecov]

Codecov Report

❌ Patch coverage is 46.19289% with 106 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.82%. Comparing base (6c8c1a6) to head (06d0051).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...es/Opc.Ua.Client/ComplexTypes/ComplexTypeSystem.cs	1.38%	71 Missing ⚠️
Stack/Opc.Ua/Types/OptionSet.cs	71.42%	21 Missing and 11 partials ⚠️
...ent.ComplexTypes/TypeBuilder/ComplexTypeBuilder.cs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3672      +/-   ##
==========================================
- Coverage   71.85%   71.82%   -0.04%     
==========================================
  Files         520      521       +1     
  Lines      115034   115230     +196     
  Branches    19411    19444      +33     
==========================================
+ Hits        82656    82761     +105     
- Misses      26799    26874      +75     
- Partials     5579     5595      +16     

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