fix: empty model name passes validation (chars().all() on empty iterator)

[jamesadevine]

Problem

generate_copilot_params validates the model name with chars().all(...), but chars().all() returns true for an empty iterator. EngineConfig::Simple("") would produce --model (trailing space, no value).

Low-severity since a blank model string is unlikely in practice, but the empty check was missing.

Fix Applied

Added model.is_empty() guard before the character validation.

Fixed in #175.

Related

• Red team audit: 🔴 Red Team Audit — High: Shell injection via engine.model + multiple YAML/expression injection vectors #171 (Finding 1 hardening)