API cleanup and architecture refinement for MediaKit image picker

[AkshayAshokCode]

Summary

This PR introduces the first phase of MediaKit API cleanup and architecture refinement for the image picker module.

The goal of this cleanup is to:

• improve API consistency
• strengthen internal architecture
• prepare the library for future cropper integration
• improve maintainability and readability
• make the public API more production-ready

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Changes Included

Builder API Cleanup

• Refactored ImagePickerBuilder
• Improved fluent API consistency
• Added clearer documentation and KDocs
• Tightened builder construction flow
• Improved readability and future extensibility

Result API Improvements

• Improved ImagePickerResult documentation
• Clarified success/cancel/error semantics
• Added notes for future sealed error migration

Error Model Cleanup

• Refined ImagePickerException
• Improved typed error consistency
• Standardized documentation and formatting
• Cleaned sealed hierarchy structure

Coordinator Refactor

• Refactored ImagePickerCoordinator
• Added lazy initialization for picker implementations
• Improved readability and separation of responsibilities
• Added future direction notes for source chooser support

Picker Implementation Cleanup

• Improved CameraImagePicker
• Improved GalleryImagePicker
• Added KDocs and internal architecture consistency
• Standardized launcher initialization

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Future Follow-up Work

Planned follow-up cleanup phases:

• stronger sealed result/error model
• lifecycle restoration improvements
• cropper module integration
• threading/background bitmap processing
• package structure cleanup
• Compose-friendly APIs
• README/sample modernization

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Notes

This PR intentionally focuses on non-breaking cleanup and architectural refinement before introducing larger API-level changes.