fix: bump Go toolchain to 1.25.8

[coderzc]

Summary

• bump the Go toolchain version in go.mod to 1.25.8
• update the test Docker image to use golang:1.25.8-alpine
• address CVE-2026-25679 and CVE-2026-27142 reported against the Go stdlib

Testing

• go test ./pkg/ctl/namespace -run '^$'

[github-actions]

@coderzc:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[nlu90]

duplicate with #2023