feat(manager/npm): use volta node pin as node version constraint

[felipecrs]

Changes

Resolves node version constraint from package.json volta node pin. If the volta node pin is present, it takes precedence over engines.node.

I originally reported:

• Renovate should detect node and npm constraints from Volta pins #34946

And back then, I had mentioned the node case, although not 100% clearly.

Turns out that the PR, which I only got to test for real now, only takes into account the npm, yarn, etc.

• feat(manager/npm): use volta pins as constraints #35869

Therefore, this PR completes the implementation by adding support for the node case.

Context

Please select one of the below:

• This closes an existing Issue: Renovate should detect node and npm constraints from Volta pins #35340
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository: https://github.com/felipecrs/renovate-volta-constraints

[felipecrs]

@viceice maybe you can merge this too?