fix(filesystem): support recursive create_directory and prevent move_file overwrites

[AbhiPra24]

Summary

Fixes #4629 and #4628.

1. create_directory parent creation (Fixes #4629)

• Problem: When create_directory is called on a path with multiple non-existent parent directories (e.g. <allowed_dir>/a/b/c), validatePath() failed on path.dirname() throwing "Parent directory does not exist: <allowed_dir>/a/b", preventing recursive directory creation even though create_directory passes { recursive: true } to fs.mkdir.
• Fix: In validatePath(), when a path does not exist (ENOENT), walk up ancestor directories until an existing ancestor is found, resolving it via fs.realpath and verifying it stays within allowedDirectories.

2. move_file non-overwriting behavior (Fixes #4628)

• Problem: move_file called fs.rename directly, silently overwriting existing destination files and causing data loss, violating the documented contract ("If the destination exists, the operation will fail").
• Fix: Added pre-rename check with await fs.lstat(validDestPath) to throw Destination already exists: <path> when target is occupied.

Verification

• Added regression tests in src/filesystem/__tests__/lib.test.ts and src/filesystem/__tests__/structured-content.test.ts.
• npm run build and npm test in src/filesystem passed (157/157 tests green).

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cc @olaservo for review when available. Thank you!