fix: re-verify connectivity when cached state is false before blocking call

[SERDUN]

Problem

On Xiaomi (Android 15 / HyperOS), after restoring the app from background:

• Signaling shows connected in the UI
• Attempting to make a call shows: "It seems you are not connected to the internet"
• Actual network IS connected

Logcat shows the socket opens at ~11:16:36, yet six seconds later the call is blocked with "Cannot create call: no network connectivity" — while the network hardware reports inetCondition = 1.

Root Cause

CallController caches connectivity state in _netConnected: bool?, updated only via connectivityService.connectionStream events.

While the app is in background, Xiaomi (OEM aggressive battery/network management) signals ConnectivityResult.none even when the network interface stays up — setting _netConnected = false. When the app returns to foreground the stream does not immediately re-emit, leaving the cache stale. The call attempt reads the stale false and blocks without re-checking actual network state.

The signaling layer is unaffected because SignalingReconnectController already handles AppLifecycleState.resumed proactively.

Fix

Only trust the cached value when it says true. When the cached value is false or null, always perform a fresh checkConnection() before blocking the call.

// Before
if (_netConnected != null) return _netConnected!;

// After
if (_netConnected == true) return true;

When genuinely offline, checkConnectivity() (native OS call) returns ConnectivityResult.none and short-circuits before the HTTP health check — no meaningful latency added for the truly-offline case.