A call to the ToolError class contained a malformed f-string; this syntax error renders the agent library unable to be imported.

This pull request fixes the issue by using single line quotes.

Steps to reproduce:

$: python
>> import agent #Should fail with unmatched f-string SyntaxError

Issue

• A line in the _handle_item function tried to raise a ToolError with a dynamic name.
• To obtain the erroring function name, a dictionary lookup is used with double quotes.
• The issue is this is nested within an f-string which also uses double quotes.

Fix

• Use single quotes for dictionary lookup.