fix: prevent symlink escape in Filesystem.contains

[jayhemnani9910]

The Filesystem.contains() function previously performed only lexical path checking, which could be bypassed using symlinks inside the project directory. An attacker could create a symlink pointing to sensitive files outside the project (e.g., ~/.ssh/id_rsa), and the file tools would allow reading them.

This fix:

• Uses realpathSync to resolve symlinks before checking containment
• Falls back to lexical check if realpath fails (e.g., file doesn't exist)
• Adds explicit handling for Windows cross-drive paths (D:\ vs C:)
• Removes the TODO comments that documented this issue