feat(gpt5): add model_verbosity for GPT‑5 via Responses API (#2108)

**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**
- [x] Code gated to GPT‑5 family only
- [x] Docs updated (`README.md`, `config.md`)
- [x] Tests added and passing
- [x] Formatting applied

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

codex-rs/config.md

@@ -243,6 +243,25 @@ To disable reasoning summaries, set `model_reasoning_summary` to `"none"` in you
 model_reasoning_summary = "none"  # disable reasoning summaries
 ```
 
+## model_verbosity
+
+Controls output length/detail on GPT‑5 family models when using the Responses API. Supported values:
+
+- `"low"`
+- `"medium"` (default when omitted)
+- `"high"`
+
+When set, Codex includes a `text` object in the request payload with the configured verbosity, for example: `"text": { "verbosity": "low" }`.
+
+Example:
+
+```toml
+model = "gpt-5"
+model_verbosity = "low"
+```
+
+Note: This applies only to providers using the Responses API. Chat Completions providers are unaffected.
+
 ## model_supports_reasoning_summaries
 
 By default, `reasoning` is only set on requests to OpenAI models that are known to support them. To force `reasoning` to set on requests to the current model, you can force this behavior by setting the following in `config.toml`: