Add typed Images client to codex-api (#23989)

## Why

Standalone image generation needs a typed `codex-api` client surface for
the Codex image proxy routes before the harness and model-facing tool
layers are wired in.

## What changed

- Added `ImagesClient` support for JSON `images/generations` and
`images/edits` requests.
- Added typed request and response shapes for generation, JSON edit
image URLs, image metadata, and base64 image outputs.
- Kept generation model slugs open-ended while requiring the generation
model field that the downstream endpoint expects.
- Exported the new client and image types from `codex-api`.
- Added coverage for generation and edit wire shapes, extra response
metadata that the client ignores, and malformed image responses missing
`data`.

## Validation

- `cargo test -p codex-api`
- `just fix -p codex-api`
- `just fmt`
- `git diff --check main`

codex-rs/codex-api/src/images.rs

@@ -0,0 +1,70 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Debug, Clone, Serialize, PartialEq)]
+pub struct ImageGenerationRequest {
+    pub prompt: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub background: Option<ImageBackground>,
+    pub model: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub n: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub quality: Option<ImageQuality>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub size: Option<String>,
+}
+
+#[derive(Debug, Clone, Serialize, PartialEq)]
+pub struct ImageEditRequest {
+    pub images: Vec<ImageUrl>,
+    pub prompt: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub background: Option<ImageBackground>,
+    pub model: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub n: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub quality: Option<ImageQuality>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub size: Option<String>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+pub struct ImageUrl {
+    pub image_url: String,
+}
+
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum ImageBackground {
+    Transparent,
+    Opaque,
+    Auto,
+}
+
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum ImageQuality {
+    Low,
+    Medium,
+    High,
+    Auto,
+}
+
+#[derive(Debug, Clone, Deserialize, PartialEq, Eq)]
+pub struct ImageResponse {
+    pub created: u64,
+    pub data: Vec<ImageData>,
+    #[serde(default)]
+    pub background: Option<ImageBackground>,
+    #[serde(default)]
+    pub quality: Option<ImageQuality>,
+    #[serde(default)]
+    pub size: Option<String>,
+}
+
+#[derive(Debug, Clone, Deserialize, PartialEq, Eq)]
+pub struct ImageData {
+    pub b64_json: String,
+}