Fix C# discriminator forwarding when base constructor parameters are empty

[Copilot]

With hierarchyBuilding, a derived constructor could implicitly call the intermediate base’s parameterless constructor, initializing the wrong discriminator and producing incorrect serialized output.

• Constructor initialization: Evaluate multi-level discriminator forwarding before checking for ordinary base parameters. Preserve the existing fallback paths.
• Regression coverage: Extend the three-level hierarchy test to cover optional-only base properties alongside required properties.

The derived constructor now explicitly passes its discriminator:

public CosmosSqlDataTransferDataSourceSink(
    string databaseName,
    string containerName)
    : base("CosmosDBSql")
{
    DatabaseName = databaseName;
    ContainerName = containerName;
}

• Fixes [http-client-csharp] hierarchyBuilding derived constructor omits discriminator when base has no required parameters #12030

[azure-pipelines]

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