Better error handling for kotlinx-coroutines-guava

[TocnoWamGovoriyYaNeHacer201145]

Better error handling for kotlinx-coroutines-guava

Changes

• Add safe completion methods to prevent uncaught exceptions in completion callbacks
• Introduce sealed class for explicit Future result state management
• Simplify cancellation logic and error propagation
• Improve exception handling with better error messages
• Maintain all original contracts and performance characteristics

Testing

• All existing tests pass without modifications
• No behavioral changes - only internal implementation improvements
• Full backward compatibility maintained

Notes

• No public API changes introduced
• Existing documentation and comments preserved
• Follows Kotlin Coding Conventions (4 spaces, no extra newlines)