…ttern

Configuration Improvements:
- **Builder Pattern**: Converted `PluginManagerConfig` from enum to struct with builder methods:
  * `path()` - set plugins directory
  * `map()` - set configuration map (replaces existing)
  * `add()` - add individual plugin configuration
  * `Default` implementation for easy initialization

- **Enhanced API**: Added new methods to `ServerContextBuilder`:
  * `plugins_config_map_add()` - add single plugin config
  * `plugins_config()` - set complete plugin config with merging

- **File Detection**: Improved file path handling with proper:
  * Path normalization (trimming trailing slashes)
  * File existence checking before attempting to load
  * Automatic `.toml` extension addition

- **Error Handling**: Better error messages for missing required configurations
- **Backward Compatibility**: Maintained all existing API functionality

Configuration Loading:
- **Priority System**: File-based configs take precedence over map-based
- **Flexible Sources**: Support both directory scanning and explicit config maps
- **Environment Integration**: Maintained environment variable support

Benefits:
1. **More Flexible**: Combine file and map configurations
2. **Easier Setup**: Builder pattern simplifies configuration
3. **Better Errors**: Clear messages for missing configs
4. **Robust File Handling**: Proper path validation and extension management