Rename the MCP connection manager to McpConnectionSet (#34708)

## What changed

- Use `McpConnectionSet` as the primary name for the collection of active MCP
  server connections throughout `codex-mcp` and its callers.
- Keep `McpConnectionManager` as a backward-compatible type alias.

GitOrigin-RevId: 9753838b4fd0a0b6429a9c4af0c077137616f388

codex-rs/core/src/session/mcp.rs

@@ -445,7 +445,7 @@ impl Session {
         let codex_apps_auth_manager =
             codex_mcp::host_owned_codex_apps_enabled(&mcp_config, auth.as_ref())
                 .then(|| Arc::clone(&self.services.auth_manager));
-        let refreshed_manager = McpConnectionManager::new(
+        let refreshed_manager = McpConnectionSet::new(
             &mcp_servers,
             mcp_config.mcp_oauth_credentials_store_mode,
             mcp_config.auth_keyring_backend_kind,