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/core/config.schema.json

@@ -2381,6 +2381,9 @@
         "allow_upstream_proxy": {
           "type": "boolean"
         },
+        "credential_broker": {
+          "type": "boolean"
+        },
         "dangerously_allow_all_unix_sockets": {
           "type": "boolean"
         },