fix(examples): handle stdin EOF gracefully in chat example

[maradini77]

Description

Fix panic in chat example when stdin reaches EOF or encounters an error.

Previously, the tokio::select! macro used pattern matching (Ok(Some(line))) which would panic if stdin.next_line() returned Ok(None) (EOF) or Err(_). This made the example crash when stdin was closed, making it unsuitable for use in scripts or automated scenarios.

The fix explicitly handles all possible outcomes of stdin.next_line():

• Ok(Some(line)) - process the line as before
• Ok(None) - gracefully exit when EOF is reached
• Err(e) - return the error instead of panicking

This ensures the example can be safely used in pipelines and automated environments.