ci(actions): update Go test matrix

[appleboy]

Summary

Update the Go test matrix to cover Go 1.26 and Go 1.27. All referenced GitHub Actions were checked against their latest available release or tag and are already current.

Related issues

• Jira: N/A
• GitHub: N/A

AI authorship

• No AI was used
• AI was used
  • Tool / model: Codex (GPT-5)
  • AI-authored files: .github/workflows/go.yml
  • Human line-by-line reviewed: None — not yet reviewed by a human.

Change classification

• Leaf change
• Core change

Plan reference

Update GitHub Actions configuration and limit the Go matrix to Go 1.26 and Go 1.27.

Verification

• Automated: Workflow YAML parsing, git diff --check, and go test ./... passed locally with Go 1.26.2.
• Manual: Confirmed the matrix contains only Go 1.26 and Go 1.27; checked every referenced Action against its latest GitHub release or tag.
• Not run: Go 1.27 tests locally; delegated to the updated GitHub Actions matrix.

Security check

• N/A - no external or security-sensitive interface changed

Risk and rollback

• Risk: CI may expose compatibility issues under Go 1.27.
• Rollback: Revert this commit to restore the previous matrix.

Reviewer guide

• Read carefully: The Go versions in the workflow matrix.
• Spot-check: GitHub Actions runs for both matrix entries.

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