Add credential brokering to network proxy feature config (#40466)

## What changed

- Add `features.network_proxy.credential_broker` and preserve structured network proxy settings when toggling the feature through config APIs.
- Protect broker enablement and provider context variables from project config, while carrying trusted context into filtered shell environments.
- Propagate brokered credentials through shell snapshots and restore real credentials for unsandboxed escalated commands.
- Normalize broker context environment keys case-insensitively on Windows and avoid conflicting remote managed-network enforcement when no proxy launch configuration is available.

## Testing

- Cover config layering, editing, project sanitization, Windows environment handling, shell snapshots, escalated execution, and end-to-end GitHub credential brokering in the zsh-fork runtime.

GitOrigin-RevId: 500e51e804e1988022052ccfe00b8fac5e495f4f

codex-rs/protocol/src/shell_environment.rs

@@ -137,6 +137,8 @@ where
 
     // Step 4 - Apply user-provided overrides.
     for (key, val) in &policy.r#set {
+        #[cfg(windows)]
+        env_map.retain(|existing, _| !existing.eq_ignore_ascii_case(key));
         env_map.insert(key.clone(), val.clone());
     }
 
@@ -248,6 +250,21 @@ mod windows_tests {
         ]);
 
         assert_eq!(result, expected);
+
+        let policy = ShellEnvironmentPolicy {
+            inherit: ShellEnvironmentPolicyInherit::All,
+            ignore_default_excludes: true,
+            r#set: HashMap::from([("gh_host".to_string(), "github.trusted.example".to_string())]),
+            ..Default::default()
+        };
+        assert_eq!(
+            populate_env(
+                make_vars(&[("GH_HOST", "github.stale.example")]),
+                &policy,
+                /*thread_id*/ None,
+            ),
+            HashMap::from([("gh_host".to_string(), "github.trusted.example".to_string(),)]),
+        );
     }
 
     #[test]