Preserve setup-node steps that use node-version-file during runtime deduplication

[Copilot]

actions/setup-node steps configured with with.node-version-file were treated as non-custom and removed by runtime deduplication. This could cause the compiler to inject a generated node-version, effectively bypassing the user’s version-file-based setup intent.

• Runtime deduplication behavior

  • Updated Node.js customization detection in DeduplicateRuntimeSetupStepsFromCustomSteps to treat non-empty with.node-version-file as user customization.
  • When present, the user-provided setup-node step is preserved and Node runtime auto-injection is filtered out for that workflow.

• Regression coverage

  • Added TestDeduplicatePreservesUserNodeVersionFile to lock expected behavior:
    • preserves user setup-node step using node-version-file
    • does not inject default node-version: '24'
    • removes duplicate generated Node setup requirement

• Illustrative case

steps:
  - uses: actions/setup-node@v6
    with:
      node-version-file: '.node-version'
      cache: yarn
  - name: Bootstrap
    run: yarn kbn bootstrap

With this change, the above step remains authoritative instead of being replaced by a generated setup step.

[Copilot]

Pull request overview

This PR fixes runtime deduplication so user-authored actions/setup-node steps that select Node.js via node-version-file are preserved instead of being replaced by generated Node setup.

Changes:

• Treat non-empty with.node-version-file on Node setup steps as customization.
• Filter the Node runtime requirement when such a customized setup step is preserved.
• Add regression coverage for the node-version-file scenario.

Show a summary per file

File	Description
pkg/workflow/runtime_deduplication.go	Updates Node setup customization detection to preserve node-version-file usage.
pkg/workflow/runtime_setup_test.go	Adds regression test covering preservation of user node-version-file setup.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 0