ci: test Go 1.26 and 1.27

[appleboy]

Summary

Run the CodeGPT test workflow against Go 1.26 and 1.27. Include the Go version in the cache key to keep compiled artifacts isolated. Migrate the credential-store dependency from the inaccessible authgate SDK to go-signet.

Related issues

• Jira: N/A
• GitHub: N/A

Architecture / flow

flowchart TD
    Event[Push or pull request] --> Matrix[Test matrix: OS x Go 1.26/1.27]
    Matrix --> Setup[setup-go selected matrix version]
    Setup --> Cache[Versioned Go cache]
    Cache --> Tests[Project test command]
    style Matrix fill:#dff0d8,stroke:#3c763d
    style Setup fill:#dff0d8,stroke:#3c763d
    style Cache fill:#dff0d8,stroke:#3c763d

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AI authorship

• No AI was used
• AI was used
  • Tool / model: Codex, GPT-5
  • AI-authored files: .github/workflows/testing.yml, go.mod, go.sum, util/credstore.go, util/credstore_test.go
  • Human line-by-line reviewed: None — not yet reviewed by a human.

Change classification

• Leaf change
• Core change

Plan reference

Keep GitHub Actions current, run Go test jobs on the Go 1.26 and 1.27 matrix, and use the accessible go-signet credential SDK.

Verification

• Automated: actionlint on the changed workflow; make test
• Manual: Reviewed the generated matrix and versioned cache key.
• Not run: GitHub-hosted Go 1.27 execution; it will run in the PR workflow.

Security check

• No secrets in the diff
• N/A - no external or security-sensitive interface changed

Risk and rollback

• Risk: The expanded matrix may expose Go-version-specific test or tooling failures.
• Rollback: Revert this single CI commit.

Reviewer guide

• Read carefully: The test matrix, setup-go version input, and cache key.
• Spot-check: Test execution remains unchanged.

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