feat: use encrypted local secrets for MCP OAuth (#27541)

## Summary

- store MCP OAuth credentials in the configured auth credential backend
- support encrypted-local OAuth storage, including legacy keyring
migration
- propagate the credential backend through MCP refresh, session, CLI,
and app-server paths

## Stack

1. #27504 — config and feature flag
2. #27535 — auth-specific secret namespaces
3. #27539 — encrypted CLI auth storage
4. this PR — encrypted MCP OAuth storage

This is a parallel review stack; the original #17931 remains unchanged.

## Tests

- `just test -p codex-rmcp-client` (the transport round-trip test passed
after building the required `codex` binary and retrying)
- `just test -p codex-mcp`
- `just test -p codex-app-server
refresh_config_uses_latest_auth_keyring_backend`
- `just test -p codex-core
refresh_mcp_servers_is_deferred_until_next_turn`
- `just test -p codex-cli mcp`
- `just fix -p codex-rmcp-client -p codex-mcp -p codex-core -p codex-cli
-p codex-app-server -p codex-protocol`
- `just bazel-lock-check`

codex-rs/protocol/src/protocol.rs

@@ -181,6 +181,7 @@ pub struct McpServerRefreshConfig {
     pub mcp_servers: Value,
     /// OAuth credential store mode to use with this server snapshot.
     pub mcp_oauth_credentials_store_mode: Value,
+    pub auth_keyring_backend_kind: Value,
 }
 
 #[derive(Debug, Clone, PartialEq)]