[test][typescript-fetch] Add / improve unit tests for current typescript-fetch oneOf logic #21057 #21464

[btpnlsl]

There is an outstanding problem related to the models generated by the typescript-fetch generator, where in certain cases the generated code is not importing models for types composed using oneOf.

The current logic related to figuring out which types are imported is located in TypescriptFetchClientCodegen.processCodeGenModel

This was originally created in #21057 to address typescript imports of primitive types & was recently touched in #21464 to address a regression related to the removal of primitive types used with oneOf and return values of the FromJSONTyped & ToJSONTyped methods.

Shortly after #21057 was merged there were reports that the PR had broken typescript imports for some users with issues #21441 and #21587 being opened.

Unfortunately the PR for #21057 contained no unit tests, so this PR attempts to add a unit test using the sample OAS provided in that PR to establish a fix for the case where imports are not included does not break the fix for the case #21057 was attempting to fix. Once the unit test is in place I am going to look into a fix for the case described in #21587.

When writing the unit test I first regressed the changes in #21057 in order to have the unit test go from red / green when the change was applied.

@DavidGrath
I also made a couple of changes to the unit test provided for #21464 in order to simplify the typescript-fetch generation and to fix the case where enum values were being tested (which was due to an error in the OAS created for testing the issue).

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Typescript: @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04) @joscha (2024/10)
Prior work:
PR #21057 - @GregoryMerlet
PR #21464 - @DavidGrath

[DavidGrath]

@btpnlsl , thank you for fixing that up

[macjohnny]

thanks for the test!