feat(cache): upgrade Redis client to v9

[asim]

Summary

• upgrade the Redis cache client from the unmaintained v8 module to the canonical github.com/redis/go-redis/v9 module at v9.22.0
• update Redis cache implementation and test imports for v9
• tidy module checksums and transitive dependencies

Testing

• go build ./...
• go vet ./...
• go test ./cache/redis
• go test ./... (environment failures: loopback connections are blocked and the credential-free AtlasCloud conformance request returned HTTP 400)
• golangci-lint run ./... (environment limitation: installed binary was built with Go 1.24 but the module targets Go 1.25.12)

Closes #4890

[Copilot]

Pull request overview

Upgrades the project’s Redis cache dependency from the deprecated github.com/go-redis/redis/v8 module to the maintained github.com/redis/go-redis/v9 module, updating the cache implementation and associated tests accordingly.

Changes:

• Switch root module dependency to github.com/redis/go-redis/v9 v9.22.0 and remove github.com/go-redis/redis/v8.
• Update cache/redis implementation and tests to import the v9 module path.
• Refresh go.sum to reflect the new dependency graph after go mod tidy.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
go.mod	Drops go-redis/v8 and adds redis/go-redis/v9 (plus indirect tidy changes).
go.sum	Updates checksums to match the v9 dependency set and tidied transitive deps.
cache/redis/redis.go	Updates Redis client import to v9 for the cache implementation.
cache/redis/options.go	Updates Redis client import to v9 for options/client construction.
cache/redis/options_test.go	Updates Redis client import to v9 for tests.

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