[core/gitstore] refactor gitstore and using expirable.LRU

[dungdm93]

Summary

• Refactor gitstore.manager repository cache using github.com/hashicorp/golang-lru/v2/expirable.
• Set default cache capacity to 100 repositories and TTL to 24 hours.
• Add functional Option helpers (WithSize, WithTTL) to configure the cache.
• Ensure eviction callback closes repository storers to release file descriptors and packfiles.
• Migrate gitstore unit tests to testify/suite (ManagerTestSuite and RepositoryTestSuite) using s.T().Context().
• Add test cases covering cache options, LRU capacity eviction, and TTL expiration.