## Why
First-party async traits should expose their `Send` contracts explicitly
without requiring `async_trait`. This completes the migration pattern
established in #27303 and #27304.
## What changed
- Replaced the remaining first-party `async_trait` traits with native
return-position `impl Future + Send` where statically dispatched and
explicit boxed `Send` futures where object safety is required.
- Kept implementations behavior-preserving, outlining existing async
bodies into inherent methods where that keeps the diff reviewable.
- Removed all direct first-party `async-trait` dependencies and the
workspace dependency declaration.
- Added a cargo-deny policy that permits `async-trait` only through the
remaining transitive wrapper crates.
- Updated `rand` from 0.8.5 to 0.8.6 to resolve RUSTSEC-2026-0097 and
keep the full cargo-deny check passing.
## Validation
- `just test -p codex-exec-server`: 216 passed, 2 skipped.
- `just test -p codex-model-provider`: 39 passed.
- `just test -p codex-core` and `just test`: changed tests passed;
remaining failures are environment-sensitive suites unrelated to this
migration.
- `cargo deny check`
- `just fix`
- `just fmt`
- `cargo shear`
- `just bazel-lock-check`
## 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`