refactor: unify handling of content examples parsing

[Mattias-Sehlstedt]

I saw #24651 that changes what request body example structure that is propagated to the mustache template. It only changed AbstractJavaCodegen rather than DefaultCodegen, so I got curious regarding why this was the case. I would expect this change to be of interest to all generators. The reason seemed to be that Java overrode the default to some extent.

I have analyzed the structure and found that the generators still have almost everything in common, and that one is basically just the extension of the other. I have thus centralized the logic in the existing ExamplesUtils, and then used that in the different implementers. The reason for this is to ensure that all generators will benefit from improvements when they are introduced (e.g., all only read the first media type, but if someone introduces so that all are read, then that is also something all generators will want to be able to utilize).

This PR also centralized the definition of the reoccurring x-example vendor extension.

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Summary by cubic

Unifies request body example parsing by centralizing logic in ExamplesUtils.getContentExample. Standardizes the x-example vendor extension with CodegenConstants.X_EXAMPLE for consistent behavior and logging.

• Refactors
  • Centralizes content example extraction in ExamplesUtils.getContentExample and uses it in DefaultCodegen and TypeScriptClientCodegen; AbstractJavaCodegen keeps model behavior (warn and ignore complex examples) and delegates to shared logic for primitives.
  • Replaces literal "x-example" with CodegenConstants.X_EXAMPLE in DefaultCodegen, AbstractJavaCodegen, AbstractFSharpCodegen, TypeScriptClientCodegen, RustServerCodegen, and RustServerCodegenDeprecated.
  • Considers only the first media type when resolving examples; the single debug log now lives in ExamplesUtils.

^{Written for commit ba4d18e. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 8 files

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