Add support for loading recursive environment variables from Container

[clue]

This changeset adds support for loading recursive environment variables from the Container. With these changes applied, factory functions may now explicitly reference their own variable name, which is especially useful for applying default values or casting or juggling string environment variables to their expected type like int or float:

$container = new Container([
    Fetcher::class => fn(string $URL, float $DELAY, Browser $browser) => new Fetcher($URL, $DELAY, $browser),
    'DELAY' => fn(float|string $DELAY = 10.0): float => filter_var($DELAY, FILTER_VALIDATE_FLOAT, FILTER_NULL_ON_FAILURE) ?? throw new \ValueError('Invalid $DELAY given'),
]);

This includes a number of updated tests to verify correct behavior and avoid introducing any potential regressions, so this has 100% code coverage and should be safe to apply. On top of this, I've updated the documentation slightly to showcase how this could be used to apply a default for the X_LISTEN environment variable. This is the next step in adding better configuration support and support for environment variables and .env (dotenv) files as discussed in #101.

Builds on top of #287, #286, #284, #184, #38, and others

[Copilot]

Pull request overview

This PR adds support for recursive environment variable loading from the Container, enabling factory functions to explicitly reference their own variable name. This is particularly useful for type casting, validation, and applying defaults to environment variables. The implementation temporarily unsets the factory reference during parameter loading to allow recursive lookups to fall through to environment variables.

Key changes:

• Modified loadVariable() to temporarily unset factory during parameter loading, enabling recursive self-references
• Updated error message in test to reflect new behavior where recursive factory references attempt environment lookup
• Added comprehensive test coverage for recursive factories with various scenarios (defaults, nullable types, error recovery)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Container.php	Core implementation: temporarily unsets factory during parameter loading to enable recursive environment variable references
tests/ContainerTest.php	Updated error message for recursive reference test; added 6 new tests covering recursive factory scenarios
docs/best-practices/controllers.md	Updated example to demonstrate recursive variable reference pattern

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