fix: reject string-like shell commands

[Aphroq]

Summary

This pull request fixes shell call command normalization when the model returns a string-like action.commands payload.

• reject top-level str, bytes, and bytearray values in coerce_shell_call() instead of treating them as generic sequences
• raise a clear ModelBehaviorError before malformed payloads can be split into character-by-character commands
• add regression coverage for string-like commands payloads while keeping valid list-based shell calls unchanged

Test plan

• uv run pytest tests/test_shell_call_serialization.py
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3091

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass