refactor: offload plugin discovery to PluginDiscovery class

[iamlasse]

What

Offloads plugin discovery logic into PluginDiscovery so Application doesn't duplicate the work. Follow-up to #44.

Also drops the ClassFileParser constructor injection that #44 introduced on PreferenceDiscovery — see "Constructor cleanup" below.

Changes

• PluginDiscovery::discoverInModule() returns PluginDefinition[] (already-parsed) instead of raw file paths.
• Extraction + loadClass + reflection moves out of Application::discoverPlugins() into PluginDiscovery.
• PluginDiscovery keeps the cheap str_contains('#[Plugin') pre-filter so non-plugin files don't pay for extraction + autoload + reflection.
• Application::discoverPlugins() simplified to iterate definitions and register.

Constructor cleanup

PluginDiscovery and PreferenceDiscovery no longer take ClassFileParser via constructor — they new one internally. The parser is a stateless utility class with one implementation that's never mocked, so the injection was ceremony: tests had to construct a parser inline just to satisfy the signature. Removing it cuts test boilerplate and avoids a public-API break for anyone who was instantiating these directly.

Application no longer threads $this->classFileParser through to either discovery class. The property remains for CommandDiscovery, which still uses it.

[markshust]

Thanks again @iamlasse — clean, well-framed PR this time, and the structural intent matches the follow-up I'd called out in #44. Pushed a follow-up commit (c61277f) and updated the PR body. Summary:

Kept and validated

• Refactor moving extraction/loadClass/reflection from Application::discoverPlugins() into PluginDiscovery. PluginDiscovery::discoverInModule() now returns PluginDefinition[] ready for registration.
• Application import cleanup follows naturally (Plugin, ReflectionClass no longer needed there).

Adjusted

• Restored the str_contains('#[Plugin') content pre-filter. Same perf concern as refactor: preference discovery and class extraction logic #44 — without it, every .php file in every module's src/ was paying for extractClassName + loadClass + ReflectionClass on each boot. The pre-filter is a cheap file read that lets non-plugin files short-circuit before the expensive work.
• Dropped the redundant class_exists($className) check after loadClass(). ClassFileParser::loadClass() already guarantees the class is loaded before returning true.

Constructor cleanup (and consistency with #44)
Dropped the ClassFileParser constructor injection from both PluginDiscovery and PreferenceDiscovery. Both now new ClassFileParser() internally. Reasoning: the parser is a stateless utility with one implementation that's never mocked — every test had to construct one inline just to satisfy the signature. Removing it cuts test boilerplate (~13 inline parser constructions in PluginDiscoveryTest) and avoids a backwards-incompatible API change for anyone who was instantiating these directly.

Application no longer threads $this->classFileParser through to either discovery class — the property stays for CommandDiscovery, which still uses it.

Tests pass (4845), lint clean. Ready to merge.