[typescript-fetch] Fix code generation for oneOf cases without discriminator

[odiak]

TL;DR

• Fix type error on generated model code using oneOf without discriminator.
• Add a sample case to verify this fix.

Detail

I improved code generation for model with oneOf without discriminator on #18702.
But later I found generated code cause type error. (But there is no problem on runtime.)
ref: comment

The cause of the problem is lack of return statement at the end of functions like MyModelFromJSONTyped.
So I added return json at the end of these functions.

PR checklist

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  Commit all changed files.
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[odiak]

Dear technical committee members of TypeScript, could you check this? 🙇
@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

[amakhrov]

thought (non-blocking):
Here, the fallback case returns the raw json. But a similar fallback in the xxxToJSON (below) returns {}.

I don't think it makes a huge practical difference (if this happens for real, one should go and fix the code to be compatible with the spec, instead of finding smart ways around it).

Just looks a bit inconsistent.

[odiak]

Indeed. I change it to return {} as any.

(I needed to add type-assertion because {} is not compatible with the return type.
Since json is also any, there is no big difference.)

[amakhrov]

Thanks @odiak - looks good to me!