Validate garm configurator max runner

[cbartz]

What this PR does

Ensure max-runner is set to non-zero value

Why we need it

GARM rejects scalesets with max-runners=0

  if s.MaxRunners == 0 {
      return fmt.Errorf("max_runners cannot be 0")
  }

  Path: parts/garm/src/params/requests.go

Checklist

• Changes comply with the project's coding standards and guidelines (see CONTRIBUTING.md and STYLE.md)
• CONTRIBUTING.md has been updated upon changes to the contribution/development process (e.g. changes to the way tests are run)
• Technical author has been assigned to review the PR in case of documentation changes (usually *.md files)
• I updated docs/changelog.md with user-relevant changes
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors
• If this PR involves Rockcraft: I updated the version
• If this PR adds/removes a charm, or changes a charm's base class, conventions, tooling, or repo structure: I updated the relevant AGENTS.md
• If this PR changes .copilot-collections.yaml or .github/instructions/: I re-checked whether the AGENTS.md "12-factor divergences" guidance still matches the upstream copilot-collections guidance