remove flag for image preparation (#29429)

## What

- make Fjord's centralized response-item image preparation unconditional
for new and resumed history
- have local user images and `view_image` outputs always defer decoding
and resizing to that path
- retain `resize_all_images` as an ignored, removed compatibility key
for released clients
- delete the flag-off producer paths and obsolete policy-specific tests

## Why

Centralized preparation is now the intended image path. Keeping the
runtime feature checks also kept two image-processing implementations
alive and allowed client config to select the legacy behavior.

This is a clean replacement for #28975, rebuilt from the latest `main`.

## How

`prepare_response_items` now runs whenever items enter history and
whenever persisted history is reconstructed. Producers emit deferred
image data, so malformed images become the existing model-visible
placeholder instead of failing the session at the producer.

## Test plan

- `just fmt`
- `just fix -p codex-core -p codex-features`
- `just test -p codex-features` — 52 passed
- focused affected `codex-core` set — 20 passed
- `just test -p codex-core handle_accepts_explicit_high_detail` — 1
passed
- full `just test -p codex-core` attempt — 2,723 passed; 88 unrelated
environment failures from read-only `~/.codex` SQLite state and
unavailable integration helper binaries

codex-rs/features/src/tests.rs

@@ -476,6 +476,23 @@ fn from_sources_ignores_removed_image_detail_original_feature_key() {
     assert_eq!(features, Features::with_defaults());
 }
 
+#[test]
+fn from_sources_ignores_removed_resize_all_images_feature_key() {
+    let features_toml =
+        FeaturesToml::from(BTreeMap::from([("resize_all_images".to_string(), false)]));
+
+    let features = Features::from_sources(
+        FeatureConfigSource {
+            features: Some(&features_toml),
+            ..Default::default()
+        },
+        FeatureConfigSource::default(),
+        FeatureOverrides::default(),
+    );
+
+    assert_eq!(features, Features::with_defaults());
+}
+
 #[test]
 fn from_sources_ignores_removed_undo_feature_key() {
     let features_toml = FeaturesToml::from(BTreeMap::from([("undo".to_string(), true)]));