[rust] support discriminators on existing fields

[ThomasVille]

Fixes #22344.

When a discriminator is specified for oneOf and the field is also a required field in one of the possible variants of oneOf,
set a default value for that field, otherwise serde will fail to parse it because it removes the tag internally.

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[ThomasVille]

I just noticed that my code injects discriminator fields that were not present before in some samples like in samples/client/others/rust/hyper/oneOf-reuseRef/src/models/apple.rs. I see there's some code to remove the discriminator and I thought it didn't work, but it looks like it does work but not always...

[ThomasVille]

It's even worse, in the oneOf-reuseRef sample, the Apple struct doesn't even have the fruitType field, so we add the field to the client even though it doesn't exist in the spec. I'll remove the code that adds the field that I took from https://github.com/OpenAPITools/openapi-generator/pull/20336/files.

[ThomasVille]

Weird, the CI fails on tests unrelated to my changes:

Error:    Run 1: TypescriptAngularArrayAndObjectIntegrationTest.generatesCorrectDirectoryStructure:65->AbstractIntegrationTest.generatesCorrectDirectoryStructure:70 [Directory content of '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-expected/model' and '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-result/model' differ.] 

Error:    Run 2: TypescriptAngularArrayAndObjectIntegrationTest.generatesCorrectDirectoryStructure:65->AbstractIntegrationTest.generatesCorrectDirectoryStructure:70 [Directory content of '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-expected/model' and '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-result/model' differ.] 

Error:    Run 3: TypescriptAngularArrayAndObjectIntegrationTest.generatesCorrectDirectoryStructure:65->AbstractIntegrationTest.generatesCorrectDirectoryStructure:70 [Directory content of '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-expected/model' and '/home/runner/work/openapi-generator/openapi-generator/modules/openapi-generator/target/test-classes/integrationtests/typescript/array-and-object-result/model' differ.] 

[ThomasVille]

@frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley Hi, I opened this PR to fix #22344. The CI fails but it seems unrelated to my changes. Can anyone help me figure out the issue?

[dsteeley]

@ThomasVille, unsure on the failure but does look to be impacting other PRs so potentially unrelated to your changes here.