fix(multi-runner): route v1 and v2 runners separately

[edersonbrilhante]

Description

• Add the experimental runner-config module consumer for v2 runner configurations.
• Skip the legacy module.runners instances when local.use_v2_config is true.
• Expose v2 runner resources through runners_map_v2.
• Add routing assertions covering stable v1 and experimental v2 configurations.

This follow-up is based on #5345 and targets its branch.

Test Plan

• terraform fmt passed for all changed files.
• Terraform initialization completed for modules/multi-runner.
• terraform validate and the filtered routing test were attempted, but the downloaded native AWS, Null, and Random providers failed plugin handshake on macOS arm64.
• Commit hooks passed Terraform fmt and merge-conflict checks. TFLint reported only the pre-existing unused iam_overrides declaration at modules/multi-runner/variables.tf:835.

Related Issues

Depends on #5345

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None