Support explicitly injecting internal RouteHandler into App or load from Container

[clue]

This changeset adds support for injecting the internal RouteHandler into the App explicitly or default to loading it from the Container. This is only used internally at the moment, but may be exposed as part of our public API in a follow-up. This also allows us to clean up the tests and avoid a number of reflection calls in our tests.

Builds on top of #290, #289, #282, #94, #42, and others

[Copilot]

Pull request overview

This PR adds support for explicitly injecting a RouteHandler into the App constructor or loading it from the Container. The primary goal is to make the framework's internal architecture more flexible while cleaning up tests by avoiding reflection calls.

Key changes:

• RouteHandler constructor now requires a Container parameter (previously optional)
• Container can now automatically instantiate RouteHandler with itself as dependency
• App constructor accepts RouteHandler instances or class names, with validation that it must be last in the middleware chain

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Io/RouteHandler.php	Made Container parameter mandatory in constructor (breaking change for internal use only)
src/Container.php	Added fallback to auto-instantiate RouteHandler with self-reference, similar to existing Container self-reference pattern
src/App.php	Added logic to accept explicit RouteHandler instances/class names and validate they appear last in middleware chain
tests/Io/RouteHandlerTest.php	Updated all tests to pass Container instance to RouteHandler constructor
tests/ContainerTest.php	Added tests verifying RouteHandler can be loaded from container with proper container injection
tests/AppTest.php	Simplified tests by injecting mocked RouteHandler directly instead of using reflection; added tests for new RouteHandler injection feature
tests/AppMiddlewareTest.php	Simplified all HTTP method tests by injecting mocked RouteHandler instead of using reflection

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

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