feat(core): Implement stop generation and state management

[inidal]

Current Behavior

1. Long text generation sometimes stops unexpectedly
2. After app restart, error appears: "Platform error: Previous invocation still processing. Wait for done=true"
3. No way to manually stop ongoing generation

Expected Behavior

1. Text generation should complete fully or be manually stoppable
2. App should handle previous generation state on restart
3. Add ability to stop generation manually

Proposed Solutions

1. Add stopGeneration() method to allow manual interruption
2. Implement proper cleanup of generation state on app lifecycle changes
3. Add state management to track and reset generation status

Technical Details

Affected components:

• FlutterGemmaPlugin (Kotlin)
• InferenceModel (Kotlin)
• flutter_gemma_interface.dart
• flutter_gemma_mobile.dart

The LLMInference API from MediaPipe should support stopping generation - this could be exposed through the plugin interface.

Impact

This would improve:

• User experience by allowing generation cancellation
• App stability by preventing hanging states
• Error handling for interrupted generations

Additional Context

This is particularly important for mobile apps where users may want to stop long generations or switch away from the app mid-generation.