[fix] restore missing pInterpolatedStringPart(), cover with test #31751

Job	Run time
Preload php-parser Order	21s
PHPStan	1m 37s
Active Classes	20s
Composer Validate	20s
Help and Version	19s
Compatible PHPStan versions	20s
Run with Space in Directory	20s
Finalize classes	25s
Check before/after test fixture on no-changes	35s
Commented Code	1m 50s
Check fixture classes are different to nodes	20s
Check no "*.php" files in rules Fixture directory	1m 43s
PHP Linter	24s
Detect composer dependency issues	22s
	9m 16s

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