Fix read file tool to respect start_line and end_line parameters

[tcdent]

Two bugs were fixed:

1. The start_line and end_line parameters were parsed but never used -
   they were marked #[allow(dead_code)] and not passed to the handler
2. Even if line filtering was implemented, line numbers would show 1, 2, 3
   instead of the actual file line numbers

Changes:

• Updated io::read_file() to accept optional start_line and end_line params
• Updated ReadFile handler to pass line range parameters
• Updated ReadFileTool::compose() to pass params from the parsed request
• Added proper tests verifying both line filtering and correct numbering