Reject empty old_string in edit tool

[fallintoplace]

Problem

old_string is accepted as empty in the edit tool. Python str.replace treats an empty string as a match between every character, which can rewrite a file at every boundary when this tool is misused.

Fix

• Add an early guard in beta_edit_tool: if old_string == "": raise ToolError("edit: old_string must not be empty").
• Preserve existing uniqueness/missing-match validation for non-empty strings.

Scope

• Only src/anthropic/lib/tools/agent_toolset.py change.
• Existing behavior for valid (non-empty) replacements is unchanged.

Testing

Not run (behavioral validation change).