Enable untrusted context tool outputs by default (#60487)

## What changed

Mark `untrusted_as_tool_output` as stable and enable it by default. Untrusted additional context, such as browser information and goal objectives, now reaches the model as `function_call_output` items in the `agent` namespace instead of user messages. Setting `features.untrusted_as_tool_output = false` preserves the message format.

## Testing

Update core, app-server, TUI, and Python SDK test expectations for the default output format. Cover context persistence, deduplication, goal continuation, and explicit opt-out behavior.

GitOrigin-RevId: 30828351e2ed584928f6055b6dcf276d8288c0fb

codex-rs/features/src/lib.rs

@@ -1283,8 +1283,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::UntrustedAsToolOutput,
         key: "untrusted_as_tool_output",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::ExecutedToolCallMetadata,