Reject string-like values for shell action commands

[mayunlei]

Summary:

• reject str/bytes/bytearray values for shell action commands in coerce_shell_call
• raise ModelBehaviorError instead of iterating character-by-character
• add regression coverage for string-like command payloads

Testing:

• uv run pytest tests/test_shell_call_serialization.py -q

[seratch]

Closing this in favor of #3092