test: validate PR 387 rebase on Go 1.26.4

[ranxi2001]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fork-only validation for the PR volcano-sh#387 rebase path after upstream PR volcano-sh#391 merged. This compares the rebased agent-sandbox v0.4.6 adaptation plus review-feedback fixes against a fork branch that mirrors upstream/main at the Go 1.26.4 baseline.

Which issue(s) this PR fixes:
Refs volcano-sh#387

Special notes for your reviewer:

• Scope: fork CI validation only; this is not intended for upstream review or merge.
• Tests already run locally: go test ./pkg/workloadmanager -count=1, make lint, go test -race ./pkg/workloadmanager -count=1, go test -race -v -coverprofile=coverage.out -coverpkg=./pkg/... ./pkg/..., non-e2e Go tests, make gen-check, go test ./test/e2e -run '^$' -count=1, make build-all, git diff --check, git diff --exit-code.
• AI assistance: Used Codex to help inspect code, validate tests, and prepare this fork-only validation PR. I reviewed and validated the changes.

Does this PR introduce a user-facing change?:

NONE