fix(filesystem): make move_file fail instead of silently overwriting the destination

[eeshsaxena]

Fixes #4628.

move_file's description and the README both say the move fails if the destination already exists:

If the destination exists, the operation will fail.

But the handler calls fs.rename directly, which silently overwrites the destination and still reports success. Because this server ships no delete tool, an agent that reads the contract reasonably treats move_file as non-clobbering, so this quietly turns into unadvertised, irreversible file deletion: move any file onto an existing target and the target's contents are gone.

Fix

Added a moveFile helper in lib.ts that checks the destination first and rejects if anything already occupies it, then renames only when the target is free:

try {
  await fs.lstat(destinationPath);
} catch (error) {
  if ((error as NodeJS.ErrnoException).code === 'ENOENT') {
    await fs.rename(sourcePath, destinationPath);
    return;
  }
  throw error;
}
throw new Error(`Destination already exists: ${destinationPath}`);

lstat (not stat) is used so an existing symlink at the destination is detected rather than followed. The move_file handler now calls this helper. Behaviour now matches the documented contract, and the successful-move path is unchanged.

Tests

Added two cases to __tests__/lib.test.ts:

• moves when the destination doesn't exist (rename is called)
• rejects with "Destination already exists" and never calls rename when the destination is present

The second test fails against the old fs.rename-only behaviour and passes with the fix. Full file suite green:

cd src/filesystem && npm run build && npx vitest run __tests__/lib.test.ts
# 48 passed