Missing --file-part or --config file is reported as a network error #96

Which area does this relate to?

• CLI code

What happened?

A missing local file exits 3 with A2ACLI_ERR_IO and the hint "check connectivity or whether the agent is running". A missing --request-payload or agent card file already exits 2 as a usage error. isIOError (internal/clierr/clierr.go:182) matches the file error because syscall.Errno satisfies net.Error.

Steps to reproduce

1. a2a send hi -a http://localhost:1 --file-part /nonexistent.txt -o json
2. Prints A2ACLI_ERR_IO and exits 3. Same for --config /nonexistent.yaml.

CLI version

main @ 609d0b5