fix(python): support self-referential object models

[adamcavendish]

Summary

• prevent generated Python httpx and requests models from importing their own
  module for recursive schema references
• share schema-aware Python type rendering while preserving the existing public
  helper paths
• add self-referential Foo and Bar golden coverage across every supported
  non-TypeScript backend

Testing

• RUSTUP_TOOLCHAIN=stable just test-all (1,289 tests)
• RUSTUP_TOOLCHAIN=stable cargo clippy --workspace --all-targets --all-features -- -D warnings
• generated-client builds for Go, Rust reqwest, Rust ureq, Rust aioduct,
  Python httpx, Python requests, Java, and Kotlin

[codecov]

Codecov Report

❌ Patch coverage is 86.15385% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/python/type_names.rs	84.21%	9 Missing ⚠️

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