chore(rust): add circular reference detection and Box<T> wrapping for recursive types

[iamnamananand996]

Description

Linear ticket: Closes FER-7724

Fixes seed failures caused by illegal circular type references in Rust SDK generation. Adds cycle detection to catch invalid recursive types early and enhances recursive type utilities to properly handle unions and undiscriminated unions.

Changes Made

• Created RustCycleDetector to detect illegal recursive cycles that cannot be represented in Rust
• Integrated cycle detection into AbstractRustGeneratorContext initialisation (fails fast on invalid schemas)
• Added visited set tracking in irUsesType() to prevent infinite recursion on circular types
• Enhanced recursiveTypeUtils.ts to handle all type shapes:
  • Discriminated unions (check all variants and base properties)
  • Undiscriminated unions (check all members)
  • Alias types (check aliased type)
• Updated seed fixtures with circular references to use Box<T> wrapping
• Updated allowedFailures in rust-model seed.yml

Testing

• Unit tests added/updated
• Manual testing completed

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