feat(sniffs): allow @phpstan directives in single-line comments

[kodyhagen-gh]

Summary

• Adds @phpstan as an exempted pattern in SingleLineCommentSniff alongside the existing phpcs: and @codeCoverageIgnore exceptions
• PHPStan ignore directives (@phpstan-ignore-next-line, @phpstan-ignore) require // syntax — they cannot be written as /* */ block comments
• Without this exception, every PHPStan ignore comment in consuming projects requires a redundant trailing // phpcs:ignore BSUWordPressCS.Commenting.SingleLineComment
• Updates SingleLineCommentUnitTest.inc with @phpstan fixture lines that are expected to pass without errors

Test plan

• vendor/bin/phpunit — all 8 tests pass
• vendor/bin/phpcs --standard=BSUWordPressCS ./Sniffs — sniff files lint clean
• Verify // @phpstan-ignore-next-line identifier -- reason is not flagged
• Verify // @phpstan-ignore identifier -- reason is not flagged