filesystem: a client Root is allowed only in its realpath form, unlike command-line directories #5183

Problem

Command-line directories and client Roots are resolved differently. resolveAllowedDirectories in src/filesystem/index.ts keeps both the spelling a directory was given in and its realpath when the two differ, so on macOS /tmp and /private/tmp are both allowed. parseRootUri in src/filesystem/roots-utils.ts keeps only the realpath of each client Root.

When a client's Roots take over, a request that uses the Root's own spelling (for example /tmp/x for the root file:///tmp, a symlinked project directory, or a Windows mapped drive that resolves to a UNC path) fails the containment check in validatePath (src/filesystem/lib.ts) with "Access denied", although it names a path inside the Root.

Expected

A Root admits both its advertised form and its resolved form, as command-line directories do. Every requested path is still realpath'd and re-checked in validatePath, so access stays bounded by the resolved directories. The POSIX symlink case can be reproduced directly; the Windows mapped-drive case is unverified. #4129 was closed as a duplicate of #235, whose fix covered command-line directories only.

Prior work

• fix(filesystem): preserve root aliases from MCP roots #4145 by @nnish16
• Preserve original root paths after realpath #4275 by @yyzquwu

The PR(s) above targeted the pre-refactor code and are closed as part of the backlog triage in #4875; this issue is where the work continues, with the PR(s) as a prototype. Thank you to their authors.