Acquire tokio mutex before spawn_blocking

[jprendes]

Avoid wasting threads waiting for mutexes to lock, do the lock operation using async instead.

[Copilot]

Pull request overview

This PR changes LoadedJSSandboxWrapper locking so the mutex is acquired asynchronously before entering spawn_blocking, reducing wasted blocking threads that would otherwise sit waiting on the lock. This fits into js-host-api’s Node-facing wrapper layer by improving how long-running guest execution serializes access to the underlying sandbox.

Changes:

• Switch LoadedJSSandboxWrapper.inner from std::sync::Mutex to tokio::sync::Mutex.
• Add helper methods (with_inner, with_blocking_inner, take_inner_with, take_blocking_inner_with) to centralize async-lock + blocking-work patterns.
• Refactor call_handler, snapshot, restore, unload, and dispose to use the new helpers.

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