Lazily create YAML data serializers

[penspanic]

Summary

• stop DataSerializerFactory from constructing YamlDataSerializer eagerly
• keep YAML support intact by creating it on the first YAML serializer request
• cover JSON-context factory lazy behavior with unit tests

Why

Tidemark's browser client uses JSON-normalized DatraRawBundle data and excludes YamlDotNet from the WASM payload. Eager construction of YamlDataSerializer caused the browser runtime to load YamlDotNet even on the JSON-only path.

Tests

• dotnet test Datra.Tests/Datra.Tests.csproj -c Release --nologo --filter "DataSerializerFactoryTests|BundleNormalizeToJsonTests|DataModelGeneratorTests"
• dotnet test Datra.Tests/Datra.Tests.csproj -c Release --nologo
• dotnet build Datra.sln -c Release --nologo