[MCP] Prompt mcp login when adding a streamable HTTP server that supports oauth (#5193)

1. If Codex detects that a `codex mcp add -url …` server supports oauth,
it will auto-initiate the login flow.
2. If the TUI starts and a MCP server supports oauth but isn't logged
in, it will give the user an explicit warning telling them to log in.

codex-rs/rmcp-client/src/auth_status.rs

@@ -44,7 +44,7 @@ pub async fn determine_streamable_http_auth_status(
 }
 
 /// Attempt to determine whether a streamable HTTP MCP server advertises OAuth login.
-async fn supports_oauth_login(url: &str) -> Result<bool> {
+pub async fn supports_oauth_login(url: &str) -> Result<bool> {
     let base_url = Url::parse(url)?;
     let client = Client::builder().timeout(DISCOVERY_TIMEOUT).build()?;