fix: VirtualUrlPlugin absolute-path virtual module IDs getting concatenated with compiler context (#20656)

When a virtual module ID is an absolute path (e.g. `virtual:C:/project/user.js`),
context "auto" incorrectly joined the derived context with `compiler.context` via
`path.join`, producing a concatenated path like `C:\cwd\C:\project`. Now absolute
contexts are detected via `isAbsolute` (from `lib/util/fs`) and used directly.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>