Strengthen tools timeout validation coverage with testify-based assertions

[Copilot]

This updates timeout validation tests for workflow frontmatter to cover key missing tools.startup-timeout and negative timeout cases. It also aligns the test style with the pkg/workflow suite’s testify assertion patterns.

• Expanded validation matrix for timeout fields

  • Added failing cases for:
    • tools.timeout negative values
    • tools.startup-timeout: 0
    • tools.startup-timeout negative values
    • tools.startup-timeout invalid non-expression string values
  • Added passing case for:
    • tools.startup-timeout as a valid GitHub Actions expression string (${{ ... }})

• Assertion style modernization

  • Replaced manual t.Fatalf / t.Errorf checks with require/assert to improve consistency and failure readability.

• Readability and intent clarity

  • Tightened subcase names to explicitly distinguish timeout vs startup-timeout behavior.
  • Added a short note explaining why the test validates via full compilation (end-to-end extraction/validation path).

{
    name: "valid startup-timeout gha expression",
    workflowMd: `
---
tools:
  startup-timeout: "${{ inputs.startup-timeout }}"
---
...`,
    shouldCompile: true,
}

• Fixes [testify-expert] Improve Test Quality: pkg/workflow/tools_timeout_validation_test.go #50363