Remove terminal resize reflow flag gates (#27794)

## Why

`terminal_resize_reflow` is now stable and should behave as always on.
Keeping the disabled runtime paths around made the feature look
configurable even though the rollout is complete, and old config could
still suggest there was a supported off mode.

## What Changed

- Marked `terminal_resize_reflow` as `Stage::Removed` while keeping it
default-enabled for compatibility.
- Ignored `[features].terminal_resize_reflow` config entries so stale
`false` settings no longer affect the effective feature set.
- Removed TUI branches that depended on the flag being disabled, so
draw, replay buffering, stream finalization, and resize scheduling all
assume resize reflow is active.
- Simplified resize smoke coverage to exercise the always-on behavior
only.

## Verification

- `just test -p codex-features`
- `just test -p codex-tui resize_reflow`
- `just test -p codex-tui initial_replay_buffer
thread_switch_replay_buffer`

codex-rs/features/src/tests.rs

@@ -32,8 +32,7 @@ fn default_enabled_features_are_stable() {
     for spec in crate::FEATURES {
         if spec.default_enabled {
             assert!(
-                matches!(spec.stage, Stage::Stable | Stage::Removed)
-                    || spec.id == Feature::TerminalResizeReflow,
+                matches!(spec.stage, Stage::Stable | Stage::Removed),
                 "feature `{}` is enabled by default but is not stable/removed ({:?})",
                 spec.key,
                 spec.stage
@@ -151,18 +150,35 @@ fn request_permissions_tool_is_under_development() {
 }
 
 #[test]
-fn terminal_resize_reflow_is_experimental_and_enabled_by_default() {
+fn terminal_resize_reflow_is_removed_and_enabled_by_default() {
     assert_eq!(
         feature_for_key("terminal_resize_reflow"),
         Some(Feature::TerminalResizeReflow)
     );
-    assert!(matches!(
-        Feature::TerminalResizeReflow.stage(),
-        Stage::Experimental { .. }
-    ));
+    assert_eq!(Feature::TerminalResizeReflow.stage(), Stage::Removed);
     assert_eq!(Feature::TerminalResizeReflow.default_enabled(), true);
 }
 
+#[test]
+fn from_sources_ignores_removed_terminal_resize_reflow_feature_key() {
+    let features_toml = FeaturesToml::from(BTreeMap::from([(
+        "terminal_resize_reflow".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());
+    assert_eq!(features.enabled(Feature::TerminalResizeReflow), true);
+}
+
 #[test]
 fn tool_suggest_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::ToolSuggest.stage(), Stage::Stable);