fix(android): re-invert background channel handshake to Dart-initiated (fixes #738)

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Summary

#738 (and #732, closed prematurely) is a handshake race, not a watchdog bug: since the July 2025 Pigeon migration, BackgroundWorker sends backgroundChannelInitialized to the Dart isolate microseconds after executeDartCallback, while the isolate is still booting. A native→Dart message that arrives before the isolate registered its Pigeon handlers is lost, so the Dart callback never runs and the worker either hangs in RUNNING forever (#732) or fails the 30s watchdog (#738). #735 only converted the symptom from hang to visible failure.

This restores the race-free, Dart-initiated handshake that worked for years before the migration:

• New host call WorkmanagerHostApi.notifyBackgroundChannelInitialized(), sent by Workmanager().executeTask after WorkmanagerFlutterApi.setUp (fire-and-forget — platforms without a native receiver must not block the task).
• Android BackgroundWorker no longer pushes anything at the isolate. It waits for the signal (routed through the plugin's existing boundWorker binding; watchdog still armed) and only then invokes executeTask down. Ordering is now guaranteed by construction: Dart can only signal once it is alive, and its handlers are registered before the signal leaves.
• Watchdog semantics unchanged: a dead engine (no signal within 30s) still fails visibly instead of hanging.

Why the old flow could fail at all: messages sent to an isolate that has not yet registered its channel handlers are dropped (no reply), which is what left workers stuck in RUNNING pre-0.10.9 and failing the watchdog in 0.10.9.

Apple platforms

Compile parity only: the new host method is implemented as a no-op because Apple workers still gate on the legacy kick (which doubles as the in-process dispatcher trigger on the main engine). The Apple headless-engine flow has the same latent race and is tracked as a follow-up (needs device validation).

Tests

• New Dart test: executeTask emits exactly one readiness signal, after handler registration, and does not wait for the native reply (task still executes while the signal is unanswered).
• Existing suite green: flutter test across all packages (workmanager 24, android 35, apple 28, linux, platform_interface, web), 63 Kotlin unit tests via :workmanager_android:testDebugUnitTest, dart analyze clean, flutter build ios --debug --no-codesign passes.

Fixes #738