fix(workflows): add missing setup-uv id in code complexity lock workflow

## Summary
- add the missing `id: setup-uv` to the `Setup uv` step in `.github/workflows/gh-aw-code-complexity-detector.lock.yml`
- keep the existing `UV_PATH: $\{\{ steps.setup-uv.outputs.uv-path }}` reference, which now resolves correctly

## Why
Issue #996 identified that this workflow referenced `steps.setup-uv.outputs.uv-path` without defining the `setup-uv` step id, causing validation/runtime wiring failures for uv exposure.

Closes #996.

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Bug Hunter](https://github.com/elastic/ai-github-actions/actions/runs/24775880387)

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> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/24775880387)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 24775880387 -n agent -D /tmp/agent-24775880387

# Fetch the bundle into a local branch
git fetch /tmp/agent-24775880387/aw-fix-996-setup-uv-id.bundle refs/heads/fix-996-setup-uv-id:refs/heads/fix-996-setup-uv-id-acc50efeceb4212d
git checkout fix-996-setup-uv-id-acc50efeceb4212d

# Push the branch to origin
git push origin fix-996-setup-uv-id-acc50efeceb4212d

# Create the pull request
gh pr create --title 'fix(workflows): add missing setup-uv id in code complexity lock workflow' --base main --head fix-996-setup-uv-id-acc50efeceb4212d --repo elastic/ai-github-actions
```