[conformance] Implement E01-E05 error handling tests

[josedab]

SDK Conformance: Error Handling Tests

Implement tests E01-E05 from streamline/tests/cross_sdk/CONFORMANCE_SPEC.md:

• E01: error_unknown_topic, E02: error_invalid_partition
• E03: error_invalid_offset, E04: error_is_retryable, E05: error_has_hint

Run: dotnet test