Add model-element unit tests to improve core coverage

[samatstariongroup]

Motivation

• Increase automated test coverage for core model/resource types that were previously untested, specifically covering Diagnostic, EFactory, and EOperation behaviors.

Description

• Add ECoreNetto.Tests/ModelElement/OperationAndFactoryTestFixture.cs containing tests for Diagnostic constructor property mapping and EFactory construction behavior.
• Add tests that validate EOperation.DeserializeChildNode null-argument guard and deserialization of an <eParameters/> child into the EParameters collection.
• Add a test that verifies EOperation.Identifier composition when an operation is attached to an EClass and include a small TestableEOperation helper to expose protected behavior for unit testing.

Testing

• Ran dotnet test ECoreNetto.Tests/ECoreNetto.Tests.csproj --nologo and all tests passed (Passed: 17, Failed: 0).
• Ran coverage collection with dotnet test ECoreNetto.Tests/ECoreNetto.Tests.csproj --nologo /p:CollectCoverage=true /p:CoverletOutputFormat=json and generated coverage.json, reporting ECoreNetto at 77.51% line coverage.
• Ran dotnet test EcoreNetto.sln --nologo to validate the full solution; all test projects completed successfully in the environment used.

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Codex Task