ArgParser: Improve missing subcommand error message

[brbzull0]

Just a decorative change.

Show attempted subcommand name in error message:

$ traffic_ctl 123
...
Error: No sub-command '123' found for traffic_ctl

Usage: traffic_ctl [OPTIONS] CMD [ARGS ...]
...

before was just:

$ traffic_ctl 123
...
Error: No subcommand found for traffic_ctl

[Copilot]

Pull Request Overview

This PR enhances error messaging in the ArgParser when a required subcommand is not found. When a subcommand is required but missing or invalid, the error message now includes the invalid subcommand name (if one was provided) to help users understand what went wrong.

• Improved error message specificity by including the invalid subcommand name when available
• Added conditional logic to differentiate between "no subcommand provided" and "invalid subcommand provided"
• Used std::ostringstream for message construction instead of simple string concatenation

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