[Bug]: edit_file hangs indefinitely when old_string is empty

Calling edit_file with an empty old_string never returns.

Reproduction:

filesystem.edit(runtimeContext, "/notes.md", "", "replacement", false);

The loop in FilesystemUtils.countOccurrences() calls indexOf("", idx).
Because the empty string has length 0, idx never advances and the loop runs forever.

Expected behavior:
Return a normal validation error such as:
"old_string must not be empty"

Suggested fix:
Validate oldString before countOccurrences() and add a unit test with a timeout assertion.

This should be a small backward-compatible fix with no API change.