feat(gpt5): add model_verbosity for GPT‑5 via Responses API

[vjain419]

Summary

• Adds model_verbosity config (values: low, medium, high).
• Sends text.verbosity only for GPT‑5 family models via the Responses API.
• Updates docs and adds serialization tests.

Motivation

• GPT‑5 introduces a verbosity control to steer output length/detail without pro
  mpt surgery.
• Exposing it as a config knob keeps prompts stable and makes behavior explicit
  and repeatable.

Changes

• Config:
  • Added Verbosity enum (low|medium|high).
  • Added optional model_verbosity to ConfigToml, Config, and ConfigProfi le.
• Request wiring:
  • Extended ResponsesApiRequest with optional text object.
  • Populates text.verbosity only when model family is gpt-5; omitted otherw
    ise.
• Tests:
  • Verifies text.verbosity serializes when set and is omitted when not set.
• Docs:
  • Added “GPT‑5 Verbosity” section in codex-rs/README.md.
  • Added model_verbosity section to codex-rs/config.md.

Usage

• In ~/.codex/config.toml:
  • model = "gpt-5"
  • model_verbosity = "low" (or "medium" default, "high")
• CLI override example:
  • codex -c model="gpt-5" -c model_verbosity="high"

API Impact

• Requests to GPT‑5 via Responses API include: text: { verbosity: "low|medium|h igh" } when configured.
• For legacy models or Chat Completions providers, text is omitted.

Backward Compatibility

• Default behavior unchanged when model_verbosity is not set (server default “
  medium”).

Testing

• Added unit tests for serialization/omission of text.verbosity.
• Ran cargo fmt and cargo test --all-features (all green).

Docs

• README.md: new “GPT‑5 Verbosity” note under Config with example.
• config.md: new model_verbosity section.

Out of Scope

• No changes to temperature/top_p or other GPT‑5 parameters.
• No changes to Chat Completions wiring.

Risks / Notes

• If OpenAI changes the wire shape for verbosity, we may need to update Respons esApiRequest.
• Behavior gated to gpt-5 model family to avoid unexpected effects elsewhere.

Checklist

• Code gated to GPT‑5 family only
• Docs updated (README.md, config.md)
• Tests added and passing
• Formatting applied

Release note: Add model_verbosity config to control GPT‑5 output verbosity via
the Responses API (low|medium|high).

[dylan-hurd-oai]

2 small comments but LGTM otherwise!

[dylan-hurd-oai]

Tested locally - confirmed this works. Thanks for adding!