fix(filesystem): handle canonical Windows share paths

[he-yufeng]

Summary

• allow filesystem requests using Windows mapped-drive paths to pass when realpath resolves inside the configured UNC allow-list
• handle UNC share allow-list entries that already end with a separator without requiring a double separator
• add Windows regression coverage for existing and new files

Fixes #4129.
Fixes #3756.

To verify

• npm exec -w src/filesystem -- vitest run tests/path-validation.test.ts
• npm exec -w src/filesystem -- vitest run tests/lib.test.ts
• npm run build -w src/filesystem
• npm test -w src/filesystem
• git diff --check

[LuuOW]

Technical audit: Verified MCP server implementation for consistency with current SDK patterns.