feat(android): replace FGS with direct WebSocket for push-bound mode

[SERDUN]

Summary

Replaces the Foreground Service hub with a direct WebSocket for `pushBound` mode on Android.
The WebSocket now runs directly in the calling isolate — identical to how iOS has always worked.
FGS is retained only for `persistent` mode.

Motivation

1. Persistent notification in push-bound mode

In the FGS hub model, Android requires a visible foreground-service notification for the entire
duration of the push isolate's lifetime — including while the user is actively on the call.
This notification cannot be suppressed while the FGS is running.
Direct mode has no foreground service in push-bound, so no persistent notification is shown.

2. Ability to fully exclude FGS from the build

With FGS only used for `persistent` mode, apps that ship exclusively with push-bound can
remove the `<service android:name=".SignalingService" .../>" declaration from
`AndroidManifest.xml` entirely. This eliminates the manifest permission surface and avoids
Android 12+ foreground-service start restrictions (`ForegroundServiceStartNotAllowedException`,
`ForegroundServiceDidNotStartInTimeException`) for push-bound users.

3. Eliminates an entire class of FGS crashes

The FGS hub introduced a set of crashes that cannot be fixed cleanly:

• `ForegroundServiceStartNotAllowedException` — Android 12+ background start restriction
• `ForegroundServiceDidNotStartInTimeException` — stop/start races during logout
• Hub polling loop, watchdog, stale port detection — all for a service the OS treats as
  disposable (`START_NOT_STICKY`, `stopSelf` on `onTaskRemoved`)

Architecture

```
Before (FGS hub):
Push → push isolate → startForegroundService() → FGS engine (WS) ← Activity

After (direct):
Push → push isolate → WebSocket (in push isolate)
Activity → WebSocket (in Activity)
```

No FGS, no IsolateNameServer hub, no background FlutterEngine (~60 MB RAM saved).

Push isolate ↔ non-push isolate handoff

Both isolates open independent WebSockets. Role detection is based purely on whether
`setHandoffCallback()` was called before `_startDirect()` — not on which isolate is running:

`_handoffCallback`	Role	Behaviour
set	Push isolate	Registers `ReceivePort` under `kPushHandoffPortName` in `IsolateNameServer`
null	Non-push isolate	On `SignalingConnected`, looks up the port and sends null

Two mechanisms close the push isolate early:

IsolateNameServer handoff — non-push isolate sends null to the push isolate's port on
`SignalingConnected`. Push isolate receives the signal → `notifyActivityTookOver()` →
`_complete()`, cancelling the reconnect timer before it fires.

4441 fast path — server sends `controllerForceAttachClose` when it detects a duplicate
session. `PushNotificationIsolateManager` handles `SignalingDisconnected(code: 4441)` as an
early-exit signal via `_complete()`.

Whichever path fires first closes the push session.

Reliability

The `SignalingModuleImpl` (WebSocket logic) is unchanged — same code, same behaviour.
Two additions on top:

• 4441 handling: follows the existing foreground-service disconnect flow — when the server
  signals that another session has taken over, the push isolate closes cleanly without treating
  it as an error.
• Handoff callback: a faster, local signal that does not depend on the server sending 4441.
  Avoids forcing the call through an error path if the server is slow or the 4441 arrives after
  the non-push isolate is already handling the call.

Files changed

File	Change
`isolate_manager.dart`	`notifyActivityTookOver()`, 4441 fast path in `SignalingDisconnected` handler; updated docs
`background_isolate_callbacks.dart`	`setModuleFactory()` registered in push isolate; `setHandoffCallback()` always registered; FGS doc references removed
`bootstrap.dart`	removed `setPushBoundStrategy()` call
`environment_config.dart`	removed `WEBTRIT_PUSH_BOUND_USE_DIRECT`
`signaling_service_platform.dart`	removed `attach()` and `setPushBoundStrategy()`
`signaling_service.dart`	removed `attach()` and `setPushBoundStrategy()` static wrappers
`constants.dart` (android)	`kPushHandoffPortName`
`plugin.dart` (android)	`_startDirect()` unconditional for pushBound; handoff port registration/cleanup; non-push isolate signal send; removed `pushBoundUseDirect` flag, `setPushBoundStrategy()`, and `attach()`
`docs/signaling_architecture_target.md`	updated dependency diagram; added handoff mechanism section