fix(ts): support discriminator properties starting with double unders…

[sidhartthhhhh]

fixes #6758

This PR fixes an issue in the TypeScript writer where properties beginning with double underscores (e.g., __component) used as discriminators in OpenAPI schemas were not properly handled in the generated models, causing TypeScript errors.

Modified TypeScriptConventionService.cs to ensure that properties with names like __component are preserved and correctly handled in composed types and serialization logic.

Specifically ensured that such properties are not renamed or stripped during type generation or access.

[baywet]

• changelog entry
• unit tests

[baywet]

this fix is super specific to this use case, it should most likely be fixed in the generation code instead.

Here is where the switch statement gets written

kiota/src/Kiota.Builder/Writers/TypeScript/CodeFunctionWriter.cs

Line 419 in 5fb1a91

writer.StartBlock($"switch ({param.Name.ToFirstCharacterLowerCase()}.{discriminatorProperty.Name.ToFirstCharacterLowerCase()}) {{");

(should use the symbol name instead, and avoid normalizing the casing)

Here is where it gets tested

kiota/tests/Kiota.Builder.Tests/Writers/TypeScript/CodeFunctionWriterTests.cs

Line 502 in 5fb1a91

Assert.Contains("switch (parentClass.odataType) {", result);