#25300 [typescript-fetch] Convert request body set to array

When a set is passed into a request body, the client will attempt to map the objects in the set to JSON, but Set doesn't have a map method. To resolve this, the set is wrapped in Array.from.

Fixes #24887

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka @joscha @KannaKim

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Summary by cubic

Fixes serialization of request bodies typed as sets in the TypeScript fetch client.

When an array body parameter has uniqueItems: true, the generated client now wraps the value in Array.from() before mapping it to JSON. Previously, the code called .map() directly on the value, which fails at runtime because Set doesn't have a map method.

• Adds a test with a new OpenAPI 3.1 spec covering object, array, and set request bodies.