Correct TypeScript deserialization for discriminated union property types

[jeffreybulanadi]

Summary

When a model property references a named oneOf schema with a discriminator (e.g. \WeatherForecast\ defined as \oneOf: [RainyDayForecast, SunnyDayForecast]\ with \discriminator.propertyName), the TypeScript deserializer was generating incorrect ??-chained subtype factory calls instead of delegating to the base type's discriminator factory function.

Root Cause

In \CodeFunctionWriter.WritePropertyDeserializationBlock, the \GetOriginalComposedType\ branch handled all composed-type properties identically by joining individual subtype factory calls with ??. This is correct for true anonymous unions (oneOf without discriminator) but wrong for named discriminated unions, where a base-type factory (\createXFromDiscriminatorValue) already handles discriminator-based subtype resolution.

Before

\\ ypescript
"forecasts": n => { weatherSummary.forecasts = n.getCollectionOfObjectValues(createRainyDayForecastFromDiscriminatorValue) ?? n.getCollectionOfObjectValues(createSunnyDayForecastFromDiscriminatorValue); },
"primaryForecast": n => { weatherSummary.primaryForecast = n.getObjectValue(createRainyDayForecastFromDiscriminatorValue) ?? n.getObjectValue(createSunnyDayForecastFromDiscriminatorValue); },
\\

After

\\ ypescript
"forecasts": n => { weatherSummary.forecasts = n.getCollectionOfObjectValues(createWeatherForecastFromDiscriminatorValue); },
"primaryForecast": n => { weatherSummary.primaryForecast = n.getObjectValue(createWeatherForecastFromDiscriminatorValue); },
\\

Changes

• \src/Kiota.Builder/Writers/TypeScript/CodeFunctionWriter.cs: added private helper \TypeHasBasicDiscriminatorInformation\ and updated \WritePropertyDeserializationBlock\ to use the base type factory when the property's composed type carries discriminator information.
• \ ests/Kiota.Builder.Tests/Writers/TypeScript/CodeFunctionWriterTests.cs: two new tests covering the collection-property and single-object-property cases, plus an end-to-end integration test.
• \ ests/Kiota.Builder.Tests/OpenApiSampleFiles/DiscriminatedUnionPropertySample.cs: new OpenAPI YAML sample for the integration test.
• \CHANGELOG.md: unreleased entry for this change.

Closes #6615