fix: address PR 387 review feedback

[ranxi2001]

Stacked validation PR for upstream volcano-sh#387.

This PR is intentionally based on feat/agent-sandbox-latest, not main, so the review surface only contains fixes for PR volcano-sh#387 feedback and CI failures. After validation, the clean fix commit can be cherry-picked/rebased back into feat/agent-sandbox-latest.

Changes:

• Align lint and test-coverage workflows with go.mod target Go 1.26.2.
• Add an explicit nil readiness watcher guard for direct sandbox creation.
• Reduce handleSandboxCreate complexity by extracting sandbox create error handling.
• Update warm-pool e2e ownership matching to use Sandbox UID for Sandbox -> Pod ownerRefs.
• Fix test fake store error propagation and deep-copy EntryPoints.

Validation:

• go test ./pkg/workloadmanager -count=1
• go test ./test/e2e -run TestNonExistent -count=1
• make lint
• go test -race -v -coverprofile=coverage.out -coverpkg=./pkg/... ./pkg/...
• go test -race ./pkg/workloadmanager -count=1
• go test ./pkg/... -count=1
• git diff --check

Known local limitation:

• go test ./... -count=1 fails only in test/e2e when local Router / WorkloadManager are not running and no kubeconfig is configured. The non-e2e package set passes.

AI assistance: Used Codex to inspect CI logs, implement the fix, and run local validation. I reviewed the changes and test output.