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/network-proxy/src/credential_broker/providers.rs

@@ -15,7 +15,7 @@ type TranslateRequestHeader = fn(&HeaderMap, &str, &str) -> Option<HeaderValue>;
 /// Providers must be declared as `static` values because the broker uses their addresses as stable
 /// identities when deduplicating credential records.
 pub(super) struct CredentialProvider {
-    context_env_vars: &'static [&'static str],
+    pub(super) context_env_vars: &'static [&'static str],
     sources: &'static [CredentialSource],
     pub(super) reset_on_configuration_change: bool,
     dummy_value: fn(&str) -> String,