fix(staker): Introduce Tier type and simplify casting in reward logic

[notJoon]

Description

This PR introduces a dedicated Tier type and refactors related code to improve type safety and reduce unnecessary casting.

Background

Previously, tier values were represented as raw uint64. This causes several issues:

• Frequent casting between uint64 and int64 when performing reward calculations (emssions and ratios are int64).
• Higher risk of accidental misuse, as any uint64 value could be passed in as a tier without validation.
• Reduced clarity when reading code that mixed tier as a nunber with tier as a sementic concept.

Changes

• Added a new Tier type (uint8)
• Changed CurrentCount to return int64 instead of int to align with reward calculation paths.
• Refactored method signatures (CurrentReward, CurrentCount, etc.) and internal logic to accept/use Tier instead of raw integers where appropriate

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[dongwon8247]

Approving. Please wait for @jinoosss's review before merging

[jinoosss]

Please verify the shape transformation function.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud