Expose system proxy support as an experimental setting (#58157)

## What changed

- Add “System proxy” to `/experimental` for opting into macOS or Windows system proxy settings. Keep `respect_system_proxy` disabled by default.
- Update doctor’s macOS proxy guidance to point to `/experimental` and `codex features enable respect_system_proxy`, and tell users to restart Codex after changing the setting.

## Testing

Add snapshots for the experimental setting and doctor guidance, plus a Responses test verifying that `x-codex-beta-features` includes `respect_system_proxy` only when enabled.

GitOrigin-RevId: caa3e1a7cbe222069f35022b06c59da578b8a4dd

codex-rs/features/src/lib.rs

@@ -1564,7 +1564,11 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::RespectSystemProxy,
         key: "respect_system_proxy",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Experimental {
+            name: "System proxy",
+            menu_description: "Use macOS or Windows system proxy settings for Codex requests.",
+            announcement: "",
+        },
         default_enabled: false,
     },
     FeatureSpec {