feat: WorkmanagerDebug handlers on web, linux and windows (same API as Android/iOS)

[ened]

Context

The debug-handler system (WorkmanagerDebug.setCurrent(...)) currently exists only on Android (Kotlin) and iOS/macOS (Swift). The web, linux and windows implementations are pure Dart, so they have no equivalent — debug information on those platforms is limited to what the developer prints themselves.

The API shape (see workmanager_android/.../WorkmanagerDebugHandler.kt):

• TaskDebugInfo(taskName, uniqueName, inputData, startTime)
• TaskResult(success, duration, error)
• WorkmanagerDebug with onTaskStatusUpdate(taskInfo, status, result) and onExceptionEncountered(taskInfo, exception)
• WorkmanagerDebug.setCurrent(handler) / getCurrent()
• Built-in handlers: LoggingDebugHandler, NotificationDebugHandler

Proposal

Add a Dart-side mirror of the same API in the workmanager package (so the call site looks identical across platforms) and emit status updates from the Dart platform implementations:

• WorkmanagerDebug.setCurrent(...) with the same handler contract (minus platform-specific context).
• LoggingDebugHandler (prints to console) + web-capable NotificationDebugHandler (browser notifications on web; no-op elsewhere).
• Wire emission:
  • web (workmanager_web): task start/completion/failure in the execution path, and dispatcher exceptions.
  • linux (workmanager_linux): in BackgroundTaskRunner.
  • windows (workmanager_windows): in the Dart execution path.

Acceptance criteria

• WorkmanagerDebug.setCurrent(LoggingDebugHandler()) works identically on web, linux, windows (and stays a no-op-friendly default).
• Custom handlers receive onTaskStatusUpdate / onExceptionEncountered with the same TaskDebugInfo/TaskStatus/TaskResult semantics.
• Android/iOS keep their existing native API unchanged.