Refactor Transition

[otherview]

Description

This PR introduces significant architectural improvements to the staker contract's transition logic, focusing on better separation of concerns, improved testability, and cleaner state management.

Key Changes

• Added transition.go: Introduces a dedicated Transition() method that handles PoS activation logic
• Reused epoch transition pattern: Transition now leverages the same epoch transition approach used in housekeeping for consistency
• Method visibility changes: Made computeEpochTransition() and applyEpochTransition() private for better encapsulation
• Improved state management: Enhanced EpochTransition struct to better represent state changes
• Cleaner transition flow: Streamlined the housekeeping process with better error handling
• Removed public AddLeaderGroup(): Eliminated direct external access to leader group manipulation

Breaking Changes

• ActivateNextValidator() method is now private (activateNextValidator())
• AddLeaderGroup() method removed from public API
• ComputeEpochTransition() method is now private (computeEpochTransition())

Fixes # (issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 26.47059% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.28%. Comparing base (49c5904) to head (58f37f0).
⚠️ Report is 2 commits behind head on release/hayabusa.

Files with missing lines	Patch %	Lines
builtin/staker/validation/service.go	0.00%	48 Missing ⚠️
builtin/staker/transition.go	40.00%	16 Missing and 8 partials ⚠️
builtin/staker/housekeep.go	80.00%	0 Missing and 2 partials ⚠️
builtin/staker/validation/linked_list.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           release/hayabusa    #1212      +/-   ##
====================================================
+ Coverage             58.24%   58.28%   +0.04%     
====================================================
  Files                   273      274       +1     
  Lines                 28966    28968       +2     
====================================================
+ Hits                  16870    16885      +15     
- Misses                10629    10647      +18     
+ Partials               1467     1436      -31     

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