fix: implement phase-2 poly-expression inference for method/construct or references (JLS §15.12.2.7)#5031
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…or references (JLS §15.12.2.7)
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After the initial type-variable inference pass (Phase 1), remaining type variables whose only source of information is a poly-expression argument (lambda or method/constructor reference) were left unresolved — for example K in groupingBy(String::new, counting()) stayed as InferenceVariable instead of resolving to String.
Add inferTypeVariablesFromPolyExpressionArguments in MethodCallExprContext: for each argument position whose AST node is a method or constructor reference, retrieve the SAM of the formal functional interface type (with Phase-1 substitutions applied), unwrap any ? extends/? super wildcard on its return type, compute the actual return type of the referenced callable (X for X::new, declared return type for Foo::bar), and match the two to derive additional type-variable bindings. Call this Phase-2 pass in both the instance-method branch and the static (NameExpr) branch of solveMethodAsUsage. Promote JavaParserFacade.toMethodUsage from protected to public to allow access from MethodCallExprContext.
Update the resolveStreamCollectGroupingByWithConstructorReference test to assert the fully resolved type Map<String, Long> instead of the previous relaxed structural checks.