Flattened struct helper functions

[Exotik850]

Add flattened content helpers render step

This PR introduces the FlattenedContentHelpersRenderStep which generates ergonomic helper accessor methods for flattened struct content patterns (e.g., pub struct Foo { pub content: Vec<FooContent>, ... } with pub enum FooContent { ... }). The step creates methods like fn bar(&self) -> Option<&String> and mutable variants thereof for each enum variant, improving usability of generated code.

Changes include:

• New render step implementation in src/pipeline/renderer/steps/flattened.rs
• Adds the step to the publicly available render steps
• Test case verifying generated helper methods for a sample schema

The feature is enabled via the renderer configuration and follows the existing pattern for optional render steps

e.g.

Config::default().with_render_step(FlattenedContentHelpersRenderStep)

[Bergmann89]

Looks good to me, nice work 👍
Some smaller things that need to be cleaned up, then this can be merged 🙃

[Bergmann89]

One last thing you forgot to revert 🙃