setProvider() updated toolsOptions.provider but didn't reload tool
instances. The sessions_spawn tool captured the old provider in its
closure at creation time, causing subagents to inherit a stale provider.
Now calls resolveTools() + setTools() after updating toolsOptions so
sessions_spawn gets a fresh closure with the correct provider.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>