Fix scoped npmMinimalAgeGate fallback

[sjh9714]

What's the problem this PR addresses?

Fixes #7192.

Scoped npmMinimalAgeGate entries currently materialize the default 1d value even when the scope leaves the setting unset. This prevents yarn config get npmScopes --json and scoped install resolution from inheriting a global disabled gate (npmMinimalAgeGate: 0).

How did you fix it?

I added a fallback setting hook for transformed configuration output, made the scoped npm gate keep undefined when unset, and wired that scoped setting to fall back to the global npmMinimalAgeGate value. I also added regression coverage for the JSON-visible config value and the scoped install behavior when the global gate is disabled.

Testing

• yarn test:unit --runInBand packages/yarnpkg-core/tests/Configuration.test.ts -t "scoped npmMinimalAgeGate"
• yarn test:unit --runInBand packages/yarnpkg-core/tests/Configuration.test.ts
• yarn build:cli
• yarn workspace acceptance-tests test:integration --runInBand sources/features/npmMinimalAgeGate.test.ts -t "unset scope gate inherits"
• yarn typecheck:all
• yarn version check
• yarn test:lint

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.