Route pet asset downloads through the shared HTTP client (#36008)

## Why

Built-in pet downloads used a standalone blocking client, so CDN redirects did
not use Codex's configured route handling.

## What changed

- Download pet spritesheets asynchronously with a reusable
  `RouteAwareClientPool`.
- Keep cache validation, asset installation, and pet decoding off the async
  runtime while sharing one asset-and-load path across startup, previews, and
  selection.
- Preserve the download size limit while consuming streamed response chunks,
  and remove the TUI's direct `reqwest` dependency.

## Testing

Add coverage for oversized response chunks, cached built-in assets, pet loads
without an existing Tokio runtime, and stale preview and selection completions.

GitOrigin-RevId: 725e76f3ddb7e84b7212769892f28dc0add15741

codex-rs/tui/src/chatwidget.rs

@@ -637,6 +637,8 @@ pub(crate) struct ChatWidget {
     review: ReviewState,
     // Active hook runs render in a dedicated live cell so they can run alongside tools.
     active_hook_cell: Option<HookCell>,
+    // Reused for built-in pet CDN requests so redirects remain route-aware.
+    pub(crate) pet_http_client: codex_http_client::RouteAwareClientPool,
     // Ambient companion rendered over the transcript area, never inside the footer rows.
     ambient_pet: Option<crate::pets::AmbientPet>,
     pet_picker_preview_state: crate::pets::PetPickerPreviewState,