Fix inherited discriminator resolution in C# model factories

[Copilot]

When three inheritance levels share a discriminator property, leaf factories select the intermediate model’s value, causing serialization to emit the wrong discriminator and base-type readers to lose the concrete type.

• Property-specific resolution: Select the most-derived value applicable to each discriminator property. Keep root and nested discriminators separate and use the declaring property’s enum mapping.
• Constructor initialization: Share resolution with factory generation and initialize inherited discriminators when intermediate constructors cannot accept them. Preserve customized get-only and private-set properties.
• Regression coverage: Cover required/optional properties, fixed/extensible enums, customized enum members, separate discriminators, and read-only customizations.

The leaf factory now selects its own enum member rather than PrometheusEnrichmentItem:

return new PrometheusInstantQuery(
    AlertsManagementType.PrometheusInstantQuery,
    additionalBinaryDataProperties: null);

The public property-specific resolver is available for downstream management constructor repair; that helper is outside this repository.

• Fixes [http-client-csharp] Model factories use intermediate discriminator for leaf models sharing an inherited discriminator #12151

[azure-pipelines]

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