fix(e2b): stabilize pagination for duplicate timestamps

[chacha923]

What this PR does

Fixes pagination for GET /v2/sandboxes and snapshot listing when multiple objects share the same timestamp sort key.

The previous cursor only stored the timestamp of the last returned item. Since claim timestamps and checkpoint creation timestamps can collide, the next page searched for the first item strictly after that timestamp and skipped remaining objects with the same timestamp.

This PR adds an optional stable cursor to the internal paginator. When a unique key is configured, pagination now sorts and resumes by (sort key, unique key), encodes the cursor as an opaque base64 JSON token, and keeps raw sort-key tokens compatible for existing clients.

Changes

• Add optional GetUniqueKey support to pkg/utils/pagination.Paginator.
• Use sandbox ID as the tiebreaker for sandbox listing.
• Use checkpoint ID as the tiebreaker for snapshot listing, with namespace/name fallback.
• Add regression coverage for duplicate sandbox claim timestamps and duplicate checkpoint creation timestamps.

Tests

go test ./pkg/utils/pagination
go test ./pkg/sandbox-manager -run 'TestSandboxManager_ListSandboxes|TestSandboxManager_ListCheckpoints'
go test ./pkg/servers/e2b -run 'TestListSandboxes_Pagination|TestListSandboxes_PaginationDuplicateClaimTime|TestListSnapshots'
go test ./pkg/utils/pagination ./pkg/sandbox-manager ./pkg/servers/e2b
git diff --check

Fixes #439

[kruise-bot]

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[kruise-bot]

Welcome @chacha923! It looks like this is your first PR to openkruise/agents 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.75%. Comparing base (c1cb28c) to head (42ca70c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
+ Coverage   79.67%   79.75%   +0.07%     
==========================================
  Files         194      194              
  Lines       13784    13836      +52     
==========================================
+ Hits        10983    11035      +52     
  Misses       2403     2403              
  Partials      398      398              

Flag	Coverage Δ
unittests	79.75% <100.00%> (+0.07%)	⬆️

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[chacha923]

Hello @furykerry @AiRanthem @zmberg ,
I wanted to politely follow up on this PR. The CI checks are passing, and I believe it is ready for review. I would appreciate any feedback whenever you have the opportunity.
Thank you!