Add models endpoint (#7603)

- Use the codex-api crate to introduce models endpoint. 
- Add `models` to codex core tests helpers
- Add `ModelsInfo` for the endpoint return type

codex-rs/Cargo.lock

@@ -858,6 +858,7 @@ dependencies = [
  "http",
  "pretty_assertions",
  "regex-lite",
+ "reqwest",
  "serde",
  "serde_json",
  "thiserror 2.0.17",
@@ -865,6 +866,7 @@ dependencies = [
  "tokio-test",
  "tokio-util",
  "tracing",
+ "wiremock",
 ]
 
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