Runtime schema generation for types (XSD, BSD, JSON Schema) + PubSub message schemas

[marcschier]

Description

Adds runtime schema generation for OPC UA data types so generated encodeable types and dynamically-added complex types can expose their schemas in every supported encoding. Stacked on the OPC UA Part 14 (PubSub) work and targets the part14pubsub branch (#3892).

Two new trimmable, NativeAOT-compatible libraries:

• Stack/Opc.Ua.Core.Schema — injectable ISchemaProvider via services.AddOpcUa().AddSchemaGeneration() (direct construction also supported). Generators, each per-type and per-namespace (with per-namespace $ref/import resolution across namespaces):
  • JSON Schema (Part 6 Annex C, draft 2020-12) — compact + verbose, faithful to the stack JsonEncoder (Int64/UInt64 → string, ByteString → base64, Float/Double → number|string, standard built-ins emitted once in $defs, unions, enums, value ranks, $ref dependency closure, and the compact SwitchField / EncodingMask members).
  • XSD — in-box System.Xml.Schema.XmlSchema object model, per-namespace xs:import.
  • BSD (OPC Binary, Part 6) — existing Opc.Ua.Schema.Binary object model serialized via a direct XmlWriter; optional-field structs use the leading 32-bit EncodingMask, unions use SwitchField, per-namespace opc:Import.
  • Resolver model: IDataTypeDefinitionResolver, in-memory DataTypeDefinitionRegistry, EncodeableFactoryDefinitionSource (reads definitions exposed by the encodeable factory) and CompositeDataTypeDefinitionResolver; thin extension API (GetXmlSchema / GetBinarySchema / GetJsonSchema / TryGetJsonSchema).
• Libraries/Opc.Ua.PubSub.Schema — AddPubSubSchema() → IPubSubSchemaProvider: per-DataSet field schemas built from DataSetMetaDataType / FieldMetaData honoring DataSetFieldContentMask, plus ua-data NetworkMessage / DataSetMessage / ua-metadata envelope schemas parameterized by the JSON content masks.

Types expose their own schemas (no manual registration required): a new IDataTypeDefinitionSource is implemented by the encodeable activators; the source generator emits a GetDataTypeDefinition override on generated data types; ComplexTypeSystem.RegisterDataTypeDefinitions(...) mirrors dynamically-built complex types; and a server-side RegisterDataTypeSchemasAsync(...) registers address-space DataType nodes.

Schemas are built in code as strongly-typed object models (no embedded schema strings), so unused generation paths trim away. Shipping code uses in-box models only — no XmlSerializer, no XmlSchema.Write, no generic JsonArray.Add<T>. JsonSchema.Net is a test-only dependency used to validate the generated schemas against real encoded OPC UA JSON.

Documentation: Docs/SchemaGeneration.md (linked from Docs/README.md).

Verification

• The whole UA.slnx solution builds with 0 warnings on net10.0; the changed libraries also build clean on net48 and netstandard2.1.
• Tests pass (net10.0): Opc.Ua.Core.Schema.Tests 64, Opc.Ua.PubSub.Schema.Tests 23, Opc.Ua.Client.ComplexTypes.Tests 2729, Opc.Ua.Server.Tests 1894, Opc.Ua.Core.Tests 3276, Opc.Ua.Types.Tests 7570, Opc.Ua.SourceGeneration.Core.Tests 3653, plus NativeAOT smoke tests in Opc.Ua.Aot.Tests (a published AOT binary was exercised).

Notes

• Both packages are added to the pack solution (UA Core Library.slnx); Opc.Ua.Core.Schema is a standalone/optional package (not a forced dependency of the Opc.Ua meta-package), matching how the PubSub packages ship.
• Known minor follow-up: ValueRanks.Any JSON is modeled as scalar-or-array (multi-dimensional arrays are accepted but not dimension-constrained).

Related Issues

Stacked on and targets the OPC UA Part 14 (PubSub) branch — #3892.

Checklist

Put an x in the boxes that apply. You can complete these step by step after opening the PR.

• I have signed the CLA and read the CONTRIBUTING doc.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added all necessary documentation.
• I have verified that my changes do not introduce (new) build or analyzer warnings.
• I ran all tests locally using the UA.slnx solution against at least .net framework and .net 10, and all passed.
• I fixed all failing and flaky tests in the CI pipelines and all CodeQL warnings.
• I have addressed all PR feedback received.

[marcschier]

Squash-merged into part14pubsub (#3892) as commit ddc11c5.