Summary

• Fix unload event not firing when watch mode restarts a script blocked on a top-level await
• Fix Node.js process.on("exit") handlers not firing on watch restart

What changed

In FileWatcherModuleExecutor::execute(), pending_unload was previously set
after execute_main_module() and dispatch_load_event(). This meant that if
the operation future was cancelled during a top-level await (the module never
finished evaluating), pending_unload stayed false and the Drop impl never
dispatched the unload event.

Now pending_unload is set before execute_main_module() and reset to
false only on module evaluation error (syntax errors, import failures). When
the future is cancelled (dropped), pending_unload remains true and Drop
correctly dispatches both the unload event and the process exit event.

The Drop impl also now calls dispatch_process_exit_event() alongside
dispatch_unload_event(), so Node.js process.on("exit") handlers fire on
restart — matching the normal shutdown path.

Towards #30912

Test plan

• Added run_watch_unload_on_restart integration test — verifies addEventListener("unload", ...) fires on file change restart
• Added run_watch_process_exit_on_restart integration test — verifies process.on("exit", ...) fires on file change restart
• Manually verified: unload fires during blocking top-level await
• Manually verified: both unload and process exit fire on Ctrl+C (after grace period)
• Manually verified: unload does NOT fire on module evaluation errors (syntax errors)

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