fix(sdk): prevent list_directory path escapes

[c8dhjp4tyv-bit]

Summary

• Normalize the project root before resolving the requested directory.
• Compare the real project root and target paths before listing.
• Reuse the shared path-containment helper so parent paths and sibling-prefix paths are rejected.
• Extend the filesystem abstraction and test mock with realpath.
• Add regression tests for the project root, valid child directories, parent traversal, sibling-prefix traversal, and symlink escapes.

Fixes #463

Validation

• bun run --cwd common typecheck
• bun run --cwd sdk typecheck
• bun run --cwd sdk test — 518 passed, 0 failed
• bun test common/src/__tests__/project-file-tree.test.ts — 12 passed, 0 failed
• git diff --check