[history server][CI] fix broken test build on master

[win5923]

Why are these changes needed?

PR #4918 and PR #4960 were developed in parallell, and #4960 was merged without rebasing onto the latest master branch. As a result, go vet now fails with the following error:

❯ go vet ./...

# github.com/ray-project/kuberay/historyserver/pkg/historyserver
# [github.com/ray-project/kuberay/historyserver/pkg/historyserver]
vet: pkg/historyserver/byte_cache_integration_test.go:34:31: undefined: utils.ClusterKey

Change

Update both test files to the current API:

• byte_cache_integration_test.go: seed clusters through a mockStorageReader plus a fake ClientManager instead of clustersMap, and request /enter_cluster/{ns}/raycluster/{name}/{session}.
• enter_cluster_test.go: pass DefaultSessionCacheMaxBytes to the five remaining NewSessionLoader calls.

Related issue number

Labels

• If this PR has user-facing changes that require documentation updates at release time, I have added the doc-updates-required label.
• If this PR contains breaking changes, I have added the breaking-change label.

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

Manual test instructions

[win5923]

cc @JiangJiaWei1103 and @chiayi to take a look when you have time. Thanks!

[CheyuWu]

nit: suggestion

Suggested change

	{Namespace: ns, Name: name, SessionName: session, OwnerKind: "rayjob", OwnerName: "job-bc"},
	{Namespace: ns, Name: name, SessionName: session, OwnerKind: utils.RayJobKind", OwnerName: "job-bc"},

[CheyuWu]

Overall, LGTM

[chiayi]

LGTM