fix: BackgroundWorker.compute hangs forever on a genuinely async callback

[pandeyshivam10]

Problem

BackgroundWorker.compute() hangs forever (never resolves, no error surfaced) when the callback passed to it does genuine asynchronous work — i.e. actually awaits something instead of completing synchronously.

Root cause

_ExecuteParams.execute() ran:

sendPort.send(callback(message));

callback returns FutureOr<R>. When it happens to complete synchronously, this works by accident, since the value is already a plain R. But when it returns a pending Future<R> (real async work), SendPort.send() throws:

Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:async' Class: _Future

A pending Future is not a sendable isolate message. This throw happens inside the worker isolate, where it's swallowed — the caller's await receivePort.first just waits forever with no error ever crossing back.

A related gap: a synchronous throw inside the callback also had no error-forwarding path, so that case hung too.

Fix

execute() now always awaits the callback (via an async closure, so sync and async callbacks are handled the same way) and sends back one of two plain, always-sendable wrapper objects instead of the raw result:

• _ComputeResult<R>(value) on success
• _ComputeError(errorString, stackTraceString) on failure — the original error/stack trace objects aren't guaranteed sendable either (arbitrary exception types can hold non-sendable fields), so only their string forms cross the isolate boundary

The caller side (_compute()) checks for _ComputeError and rethrows a _WorkerComputeException with the reconstructed stack trace; otherwise it unwraps _ComputeResult.value.

Testing

Added test/background_worker_test.dart covering:

• the original synchronous case (no regression)
• a callback that does genuine async work (real await)
• a callback with multiple sequential awaits
• a callback that throws synchronously
• a callback that throws asynchronously (after an await)

All pass. dart analyze is clean, no regressions in the existing suite.