Addresses code quality review of 9161868:
- PluginDiscoveryResult.Error is now Exception? rather than string?,
preserving stack traces across the plugin boundary for logging
- PluginDiscovery.Scan orders subdirectories by ordinal string comparison
so plugin load order is reproducible across platforms
Addresses code quality review of c082ecf:
- Require takes a literal JSON field name, no more fragile PascalCase->camelCase transform
- Parse_MissingRequiredField_Throws asserts exact message, not substring
- Remove unused using System.Text.Json.Serialization
