Address code review feedback on _parse_response:
1. Restore isinstance(raw, str) guard so non-string message/reason values
(e.g. integers, lists) from a malformed hook response fall back to the
default rather than being forwarded as-is. This keeps the contract that
message in the returned dict is always a string.
2. Extract the repeated literal 'Blocked by shell hook.' into a module-level
constant _DEFAULT_BLOCK_MESSAGE to avoid duplication and make it easy to
change in one place.
Four new unit tests added to tests/agent/test_shell_hooks.py covering:
- action block with no message (uses default)
- decision block with no reason (uses default)
- action block with empty string message (uses default)
- action block with non-string message, e.g. integer (uses default)
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