fix(filesystem): resolve symlinked allowed directories to both forms

[wingding12]

Summary

Fixes #3253

Problem
On macOS, /tmp is a symlink to /private/tmp. When users specify /tmp as an allowed directory:

1. Server startup resolves /tmp to /private/tmp and stores only /private/tmp in allowedDirectories
2. When user requests /tmp/file.txt, the initial validation check fails because /tmp doesn't match /private/tmp
3. User gets "Access denied - path outside allowed directories" error

Solution
Store both the original normalized path andthe resolved path in allowedDirectories. This allows users to access files through either form:

• /tmp/file.txt matches /tmp in allowedDirectories
• /private/tmp/file.txt matches /private/tmp in allowedDirectories

Changes

• src/filesystem/index.ts: Modified allowed directory initialization to store both original and resolved paths when they differ
• src/filesystem/__tests__/path-validation.test.ts: Added test for the symlink behavior

Tests

• Added new test "allows paths through both original and resolved symlink directories"
• Verified fix works on macOS by observing both paths in allowedDirectories output

Reproduction

Before fix:

npx -y @modelcontextprotocol/server-filesystem /tmp
# Then request /tmp/test.txt -> REJECTED

After fix:

npx -y @modelcontextprotocol/server-filesystem /tmp
# Server logs: allowed directories: ['/tmp', '/private/tmp']
# Then request /tmp/test.txt -> ACCEPTED