feat: add dynamic nested access to plugin options

Completes Phase 5 of the CLI refactoring plan by enabling nested
plugin option access (e.g., options.tesseract.timeout) alongside
the legacy flat access (options.tesseract_timeout).

Changes:
- Add module-level plugin option model registry in _options.py
- Add __getattr__ to OCROptions for dynamic namespace access
- Register plugin models in setup_plugin_infrastructure()
- Add test for nested plugin option access

Plugin option instances are lazily created from flat field values
and cached for subsequent access.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ocrmypdf/api.py

@@ -115,10 +115,15 @@ def setup_plugin_infrastructure(
 
     # Let plugins register their option models
     option_models = plugin_manager.hook.register_options()  # pylint: disable=no-member
+    all_plugin_models: dict[str, type] = {}
     for plugin_options in option_models:
         if plugin_options:  # Skip None returns
             for namespace, model_class in plugin_options.items():
                 registry.register_option_model(namespace, model_class)
+                all_plugin_models[namespace] = model_class
+
+    # Register plugin models with OCROptions for dynamic nested access
+    OCROptions.register_plugin_models(all_plugin_models)
 
     # Store registry in plugin manager for later access
     plugin_manager._option_registry = registry