[WIP] test: archive agent-sandbox v0.2.1 adaptation

[ranxi2001]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Fork-only archival validation for the staged agent-sandbox compatibility study.

This PR adapts the current AgentCube baseline from sigs.k8s.io/agent-sandbox v0.1.1 to v0.2.1. The code remains source-compatible, so the diff is limited to go.mod and go.sum.

This is not an upstream review request. It exists to preserve the v0.2.1 checkpoint and trigger fork CI.

Which issue(s) this PR fixes:
Refs volcano-sh#386

Special notes for your reviewer:

• Scope: agent-sandbox v0.2.1 dependency-only checkpoint.
• Changed files: go.mod, go.sum.
• CI result: all fork checks passed on head a4506d6, including build, coverage, e2e-test, golangci-lint, Codegen Check, Python Lint, codespell, and python-sdk-tests.
• Tests:
  • go test ./pkg/workloadmanager ./cmd/workload-manager ./cmd/agentd -count=1
  • go list ./... | grep -v '^github.com/volcano-sh/agentcube/test/e2e$' | xargs go test -count=1
  • go test ./test/e2e -run '^$' -count=1
  • make build-all
  • make gen-check
  • git diff --check
• Note: no fresh k3s/k3d runtime e2e was run for this checkpoint.
• AI assistance: Used Codex to run the staged compatibility experiment and prepare this fork-only PR. I reviewed the changes and validation output.

Does this PR introduce a user-facing change?:

NONE