pydantic app-server type generation

[sdcoffey]

Summary

Note to reviewer: 80% of the diff is generated pydantic types

This PR adds Python/Pydantic code generation for the app-server protocol in codex-rs/app-server-protocol, using the existing JSON Schema export layer rather than introducing new Rust derive macros. The goal is to make the Rust protocol types the single source of truth while generating Python bindings that stay aligned with the existing TypeScript and JSON schema artifacts.

The implementation is intentionally scoped to code generation only. It does not include PyPI packaging or release automation.

What Changed

The protocol exporter now generates Python bindings alongside the existing TypeScript and JSON outputs. A new codex app-server generate-python CLI subcommand was added, and schema fixture regeneration now vendors Python output under schema/python/.

The generated Python package preserves the existing protocol split:

• root/shared models live in codex_app_server_protocol.models
• v2-specific models live in codex_app_server_protocol.v2.models

To improve readability of the generated Python, the exporter now enables title-based naming in datamodel-code-generator. On top of that, Python-only schema title rewrites are applied for request/notification envelope wrappers so they generate stable names like ItemStartedServerNotification instead of collision-driven suffixes such as ItemStartedNotification1.

The generated base model is also patched to use populate_by_name=True, which keeps aliased fields such as _meta working correctly in Pydantic.

Design Notes

This PR deliberately keeps Python concerns in the export layer rather than pushing them into Rust protocol derives. That avoids coupling the Rust model definitions to Python packaging or Pydantic-specific behavior and keeps ts-rs focused on TypeScript.

A generic schema-to-Pydantic generator is still being used under the hood, so this approach is best viewed as a pragmatic codegen path rather than a formal compatibility proof. The exporter now does additional shaping of the Python input schema to preserve version boundaries and improve generated names, which gets us much closer to a usable generated API without introducing a separate custom emitter.

Validation

• Regenerated vendored schema fixtures, including Python output
• Ran cargo test -p codex-app-server-protocol
• Ran just fix -p codex-app-server-protocol
• Ran just fmt

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[chatgpt-codex-connector]

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[sdcoffey]

I have read the CLA Document and I hereby sign the CLA

[sdcoffey]

closing for #14232