Respect CoversNothing in Cest coverage - #6947

Summary

• preserve class- and method-level #[CoversNothing] metadata for Cest tests
• discard coverage collected for those tests before php-code-coverage converts target arrays
• add focused regression coverage for the sentinel and the coverage end path

Root cause

For PHPUnit 10 and newer, Cest coverage target extraction returned an empty target list for CoversNothing, losing the historical false sentinel. Restoring that sentinel exposed a second issue in php-code-coverage 12 and newer, where it was passed to the array-only TargetCollection::fromArray() API.

Verification

• php codecept run unit Codeception/Test/CestTest --no-colors
• php codecept run unit Codeception/Test --no-colors
• full unit suite: 260 tests, 513 assertions, 6 skipped
• focused PHPStan analysis of both changed production files
• production and test PSR-12 checks
• PHP syntax checks and git diff --check

Fixes #6940