feat(core): set default HTTP user agent

[kgrg]

Bug description

Yarn HTTP requests currently fall back to got's default User-Agent, which identifies the HTTP client rather than Yarn itself. This makes it difficult for sysadmins to distinguish Yarn traffic or enforce package-manager/version allow-lists.

Closes #7146

Root cause

httpUtils.request forwarded caller-provided headers directly to got. When callers didn't provide a User-Agent, got supplied its own default header.

Fix

• Add a default User-Agent header for Yarn HTTP requests in the form yarn/<version> node/<node-version>
• Use the bundled Yarn version when available, falling back to the core package version for source/test environments
• Preserve custom User-Agent headers when callers/plugins explicitly provide one
• Cover the default and custom-header behavior in httpUtils unit tests

Verification

• yarn test:unit packages/yarnpkg-core/tests/httpUtils.test.ts --runInBand
• yarn typecheck:all
• git diff --check

Note

I also tried yarn test:lint, but it failed before linting the changed files with an ESLint config-loader error: TypeError: Cannot convert undefined or null to object.

Risk

Low. The change only adds a default header when no User-Agent is already configured; explicit caller-provided values are left unchanged.