fix: correct pubkey auth response, strncpy warning, and NUL byte validation

- auth_pubkey: return SSH_AUTH_SUCCESS for key offers instead of
  SSH_AUTH_PARTIAL, which incorrectly signals partial authentication
- command history: replace strncpy with snprintf to eliminate
  -Wstringop-truncation warning on GCC
- utf8_is_valid_sequence: reject NUL byte (0x00) in single-byte
  validation to prevent C string truncation attacks

Closes #34

.gitignore

@@ -8,6 +8,3 @@ host_key.pub
 .DS_Store
 test.log
 *.dSYM/
-tests/unit/test_utf8
-tests/unit/test_message
-tests/unit/test_chat_room