Move LinkedList to it's own service

[otherview]

Description

This PR refactors the staker linked list implementation to separate linked list operations from validation storage, improving performance and maintainability by eliminating redundant storage writes.

Key Changes

• New linked_list package: Created dedicated package at builtin/staker/linked_list/ for linked list operations

• Separated storage concerns: Linked list now manages its own Next/Prev pointer storage independently from validation data

• Clean separation: Validation struct no longer contains Next/Prev pointers

• Eliminated redundant writes: Removed the 3-write problem where linked list operations required updating multiple Validation records

• Focused storage operations: Linked list operations now only touch pointer storage, not validation data

• Maintained API compatibility: External interfaces remain unchanged

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 44.51613% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.25%. Comparing base (49c5904) to head (e9e8741).
⚠️ Report is 3 commits behind head on release/hayabusa.

Files with missing lines	Patch %	Lines
builtin/staker/linkedlist/linked_list.go	55.26%	35 Missing and 16 partials ⚠️
builtin/staker/validation/service.go	0.00%	32 Missing ⚠️
builtin/staker/staker.go	66.66%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release/hayabusa    #1217   +/-   ##
=================================================
  Coverage             58.24%   58.25%           
=================================================
  Files                   273      274    +1     
  Lines                 28966    28963    -3     
=================================================
+ Hits                  16870    16871    +1     
- Misses                10629    10653   +24     
+ Partials               1467     1439   -28     

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[paologalligit]

Looks good to me, left a minor comment

The base branch was changed.