Support custom runner with new X_EXPERIMENTAL_RUNNER variable

[clue]

This changeset adds support for using a custom runner with a new X_EXPERIMENTAL_RUNNER environment variable. The goal is to allow custom application runners, think queue runners, CRON, a FastCGI server and more.

class AcmeRunner
{
    public function __invoke(callable $app): void
    {
        // 1. receive request
        // 2. call $app($request)
        // 3. send response
    }
}

$container = new FrameworkX\Container([
    // 'X_EXPERIMENTAL_RUNNER' => AcmeRunner::class
    // 'X_EXPERIMENTAL_RUNNER' => fn(bool|string $ACME = false): ?string => $ACME ? AcmeRunner::class : null
    'X_EXPERIMENTAL_RUNNER' => fn(?string $X_EXPERIMENTAL_RUNNER = null): ?string => $X_EXPERIMENTAL_RUNNER,
]);

$app = new FrameworkX\App($container);

Note that this is an experimental feature and the API may be subject to change in future releases. 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 implemented in #294/#236 should be good enough.

Builds on top of #294, #292, #289, #288, #287, #236, #224, #44, and others

[Copilot]

Pull request overview

This PR adds support for custom application runners through a new experimental X_EXPERIMENTAL_RUNNER environment variable. This enables users to provide custom runners for advanced use cases like queue runners, CRON jobs, FastCGI servers, and other specialized application execution patterns.

Key changes:

• Introduced X_EXPERIMENTAL_RUNNER environment variable support in Container::getRunner()
• Added runtime validation to ensure custom runners are callable
• Comprehensive test coverage for the new functionality including error cases

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Container.php	Modified getRunner() to check for X_EXPERIMENTAL_RUNNER environment variable and validate that returned runner is callable
src/App.php	Updated $runner property type annotation and documentation to reflect support for custom callable runners
tests/ContainerTest.php	Added three new test cases for custom runner functionality and updated existing test mocks to account for the additional X_EXPERIMENTAL_RUNNER check
tests/AppTest.php	Added integration test to verify custom runner is properly invoked via App::run()
docs/best-practices/controllers.md	Added documentation example showing how to configure X_EXPERIMENTAL_RUNNER environment variable

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

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