fix: network status crashes

[gastonfournier]

Context

• Fixes Android 11 binder race (SecurityException) when ConnectivityManager.registerNetworkCallback is invoked on startup (Android 11 ConnectivityManager crash #140).
• Hardens shutdown to avoid IllegalArgumentException: NetworkCallback was already unregistered when clients call DefaultUnleash.close() defensively (NetworkCallback was already unregistered when calling DefaultUnleash.close() #139).

Fixes #139
Fixes #140

What’s Changed

• Added bounded retry with main-thread scheduling for network callback registration, plus an epoch guard so stale retries no-op after close() runs (NetworkStatusHelper).
• Track callbacks only once registration succeeds; stale callbacks get unregistered immediately if the epoch shifts.
• close() now clears tracked callbacks, catches redundant unregisterNetworkCallback exceptions, and suppresses security errors so repeated close calls are safe.
• Tests expanded to cover retry success/failure, cleanup after close, and multiple-close scenarios under the new safeguards.

Testing

• Manual validation on API 30 emulator:
  • No exception detected but debugged to validate it handles availability and loss of network connection.
  • Manually simulated a SecurityException and verified the retry

[chriswk]

Looks reasonable. I say go with it.