Rename internal SAPI handlers to HttpServerRunner and SapiRunner and update API to use invokable classes

[clue]

This changeset renames the internal SAPI handlers to HttpServerRunner and SapiRunner and updates the API to use invokable classes instead of a run() method. This is mostly done as an internal preparation at the moment, but may be exposed as part of our public API that allows us to reuse this logic for more classes in a follow-up.

The goal is to prepare the internal APIs to allow custom application runners, think queue runners, CRON, a FastCGI server and more. Once PSR-15 is fully integrated (see wish list #293), we may decouple this further to use the PSR-15 RequestHandler API. For now, the callable syntax should be good enough.

Builds on top of #292, #289, #236, #224, #44, and others

[Copilot]

Pull request overview

This PR refactors internal application runners by renaming classes and converting them to use invokable syntax. The changes prepare the codebase for future extensibility with custom application runners (queue runners, CRON, FastCGI, etc.).

• Renamed SapiHandler to SapiRunner and ReactiveHandler to HttpServerRunner
• Changed from run() method to __invoke() invokable class pattern
• Moved classes from FrameworkX\Io namespace to FrameworkX\Runner namespace

Reviewed changes

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Show a summary per file

File	Description
tests/Runner/SapiRunnerTest.php	Updated test class name, namespace, and test methods to use invokable syntax instead of run() method
tests/Runner/HttpServerRunnerTest.php	Updated test class name, namespace, and test methods to use invokable syntax with added exception placeholders in handlers
tests/ContainerTest.php	Updated test method names from getSapi* to getRunner* and updated class references throughout
tests/AppTest.php	Updated property references from $sapi to $runner and test method expectations for invokable syntax
src/Runner/SapiRunner.php	Renamed class, moved to Runner namespace, changed to invokable with __invoke() method, updated documentation
src/Runner/HttpServerRunner.php	Renamed class, moved to Runner namespace, changed to invokable with __invoke() method, added required imports, updated documentation
src/Container.php	Updated to use new class names and getRunner() method instead of getSapi()
src/App.php	Updated property name to $runner, updated invocation syntax to use callable syntax, updated documentation references

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