feat: allow configuring Pest configuration discovery paths

[wanted80]

PestFileDiscoverer currently searches PHPStan's analysis paths and the entire working directory recursively for Pest.php files. Large data directories and mounted storage beneath the project root are therefore traversed even when they are outside PHPStan's analysis paths.

Add an optional pest.configPaths setting so projects can restrict configuration discovery to their test directories:

parameters:
    pest:
        configPaths:
            - %currentWorkingDirectory%/tests

The setting controls Pest.php discovery only. PHPStan continues to analyse its configured paths. Ancestor Pest.php files are still discovered up to the project root, and discovery remains memoized.

• null preserves the current default search through analysis paths and the working directory.
• A list searches only those paths and their ancestors.
• An empty list disables configuration discovery.

The parameter has a schema and is passed through the existing discovery service. Tests cover restricted paths, multiple directories, an empty list, the unchanged default, ancestor configuration and real PHPStan service wiring.

Validation:

• composer test:lint: Rector and Pint pass.
• composer test:types: PHPStan at maximum level reports no errors.
• composer test:unit: 523 passed tests and two notices in unchanged PHPStan Scope mock tests; exit code 0.
• Focused discovery tests: 12 passed, 26 assertions.

[MrPunyapal]

Could we use a unique temporary directory here instead of the fixed pest-discovery-container path?

The test suite runs with pest --parallel, so a fixed directory could be shared by concurrent test runs or leave state behind between runs.

[wanted80]

I hope I got it right, because my brain is a little fried tonight. XD