fix: handle SDK disposal gracefully to prevent crashes on app exit

[DeckerSU]

Fixes unhandled exceptions that occur when the application is closed while widgets and BLoCs are still accessing a disposed KomodoDefiSdk instance. This ensures graceful error handling instead of crashes during app shutdown.

Related Issues

Fixes #3341
Related to #3339

⚠️ This PR only fixes the issues with lastKnownUsdBalance and _listenToAuthStateChanges mentioned in #3341. Due to the app architecture, similar errors can occur elsewhere when exiting the application, where the SDK has already been disposed but some components, widgets, or BLoCs are still trying to access it.

p.s. If this PR is considered for merging, we should also consider merging #3339, which fixes another set of errors that occur on exit in Linux:

embedder.cc (2572): 'FlutterEngineRemoveView' returned 'kInvalidArguments'. Remove view info was invalid. The implicit view cannot be removed.

** (KomodoWallet:558118): CRITICAL **: 20:57:49.660: FlOpenGLManager *fl_engine_get_opengl_manager(FlEngine *): assertion 'FL_IS_ENGINE(self)' failed
Segmentation fault (core dumped)

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch patch-fix-sdk-disposal-handling-on-exit

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[github-actions]

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[smk762]

Caught same error as noted in linked issue after exit while logged in (during initial activation storm).