[typescript-fetch] Rework fix infinite recursion with self-referencing discriminator mappings

[sermler]

Reworks of #21956
Fixes #21956 (comment)
Adds fixture sample requested in #21956 (comment)

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[wing328]

please merge the latest master to your branch 🙏

[joscha]

Adds fixture sample requested in #21956 (comment)

thank you!

[sermler]

done

[wing328]

thanks for the quick fix.

let's give it a try