refactor(cache): extract shared redis base and fetch-through helper

[appleboy]

Summary

• Extract duplicated Set, MSet, Delete, Close, and Health methods from RueidisCache and RueidisAsideCache into an embedded redisBase[T] struct, eliminating ~60 lines of copy-paste
• Extract identical cache-aside logic (Get → fetch → Set) into a shared fetchThrough[T] helper, used by both MemoryCache and RueidisCache
• Net reduction of 56 lines with no behavior change

Test plan

• All 20 existing cache tests pass (go test ./internal/cache/)
• Full project builds cleanly (go build ./...)
• Verify Redis-backed caches work in integration environment

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 13.33333% with 65 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/cache/redis_base.go	0.00%	43 Missing ⚠️
internal/cache/rueidis_aside.go	0.00%	16 Missing ⚠️
internal/cache/rueidis.go	0.00%	6 Missing ⚠️

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