Add unit tests for ContainmentUpdater, ContainerList, and StructuralFeatureHelper

[samatstariongroup]

Motivation

• Improve unit-test coverage around containment semantics and container collections to prevent regressions in reparenting and removal behavior.
• Validate Handlebars helpers for EStructuralFeature to ensure block-helper rendering and type-name resolution behave as expected.

Description

• Added ECoreNetto.Tests/ModelElement/ContainerListTestFixture.cs which tests ContainerList<T> behavior including Add, AddRange, and indexer setter validations and re-parenting.
• Added ECoreNetto.Tests/Utils/ContainmentUpdaterTestFixture.cs which tests ContainmentUpdater.RemoveFromContainer for null input, no-container no-op, supported-type removals, and unsupported-type exception path.
• Extended ECoreNetto.HandleBars.Tests/StructuralFeatureHelperTestFixture.cs with tests for the StructuralFeature.IsEnumerable block helper and StructuralFeature.QueryTypeName success and invalid-context exception cases.

Testing

• Attempted to run the test project with dotnet test ECoreNetto.Tests/ECoreNetto.Tests.csproj --no-restore but execution failed because dotnet is not installed in this environment (bash: command not found: dotnet).
• No automated tests were executed here; the added tests should be run by CI or a local environment with the .NET SDK to verify success.

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Codex Task