[CodeQuality] Fix AddInstanceofAssertForNullableArgumentRector duplicate assertInstanceOf inside traits

[TomasVotruba]

Fixes rectorphp/rector#9863

Problem

Inside a test trait, AddInstanceofAssertForNullableArgumentRector is not idempotent. Every rector process run appends one more identical $this->assertInstanceOf(...) line, forever.

The rule's only cross-run guard was implicit: the inserted $this->assertInstanceOf() narrows the variable's type, so SimpleTypeAnalyzer::isNullableType() stops matching on the next run. In a class extending TestCase that holds. In a trait $this is not a PHPUnit\Framework\Assert, so the assert never narrows, the variable stays nullable, and a fresh assert is appended each run.

 $someObject = $someFactory->create();
 $this->assertInstanceOf(SomeClass::class, $someObject);
+$this->assertInstanceOf(SomeClass::class, $someObject);   // added again every run
 $someFactory->process($someObject);

Fix

Stop depending on type narrowing for idempotency. While scanning statements, detect a pre-existing assertInstanceOf(<Type>::class, $variable) and drop that variable from the nullable collection, so no second assert is spliced in. Works in both classes and traits, and hardens the class case when the PHPUnit type-specifying extension is unavailable.

Tests

• skip_already_asserted_in_trait.php.inc — trait already carrying the assert stays unchanged (fails before the fix: a duplicate assert is added).
• nullable_arg_in_trait.php.inc — the rule still fires once in a trait.

[TomasVotruba]

Let's ship it 👍