fix(napi): ensure tokio runtime is initialized for dlopen

[Brooooooklyn]

Summary

• Fixes async functions panic when using nodejs dlopen (Async functions panic when using nodejs dlopen. #2847)

Problem

When modules are loaded via dlopen() instead of require(), async functions would panic with:

thread '<unnamed>' panicked at /home/user/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/napi-3.2.0/src/tokio_runtime.rs:113:6:
Access tokio runtime failed in spawn

In the repro, the exports object was GC after the file is imported:

// loader.mjs

import { join } from "node:path";
import { dlopen } from "node:process";
import { fileURLToPath } from "node:url";

const nativeBinding = { exports: {} };
dlopen(nativeBinding, join(fileURLToPath(import.meta.url), "..", `module.linux-x64-gnu.node`));

globalThis.hello = nativeBinding.exports.hello;

This causes the tokio runtime to shut down incorrectly. So we need to add cleanup hook via napi_add_env_cleanup_hook when available.

Fixes #2847

🤖 Generated with Claude Code

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[Brooooooklyn]

cursor review

[Copilot]

Pull Request Overview

This PR fixes a runtime panic that occurs when using async functions in NAPI modules loaded via dlopen() instead of require(). The issue was that the tokio runtime wasn't being initialized in dlopen scenarios, causing async functions to panic when trying to access an uninitialized runtime.

• Adds start_async_runtime() calls to all runtime access functions to ensure proper initialization
• Fixes panics in spawn(), block_on(), spawn_blocking(), and within_runtime_if_available() functions
• Includes explanatory comments documenting the dlopen scenario requirement

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

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