test: add comprehensive tests for error types and SeqNum

Cover Display, From conversions, and io::Error mappings for all
error types in the types crate: MetaNetworkError, MetaAPIError,
MetaClientError, MetaError, MetaStartupError, ForwardRPCError,
MetaOperationError, and SeqNum. These types previously had 0-38%
test coverage.

Changes:
- Add `error_types` integration test module with 58 tests
- Test `name()`, `is_retryable()`, `add_context()` helper methods
- Verify io::ErrorKind mappings for all error-to-io conversions
- Test raft error conversions (ClientWriteError, Fatal, ChangeMembership)

Author: Zhang Yanpo