Migrate agent identity to the shared HTTP client (#34631)

## Why

`codex-http-client` is the intended owner of direct `reqwest` usage.

## What changed

- Route agent identity registration, task registration, and JWKS requests through `HttpClient`.
- Preserve retry classification using `HttpError` and `http::StatusCode`.
- Remove `codex-agent-identity` from the temporary direct-`reqwest` dependency exceptions.

GitOrigin-RevId: 7445c9f2521e6d8ef0969a1483c81f797bf91316

codex-rs/deny.toml

@@ -239,7 +239,6 @@ deny = [
         # Intended owner.
         "codex-http-client",
         # Temporary migration exceptions.
-        "codex-agent-identity",
         "codex-api",
         "codex-app-server",
         "codex-core",