[PHPStan] Add PreferGenericTemplateOnReturnedUnionParamRule to spot hidden generics

[TomasVotruba]

Adds a self-analysis PHPStan rule that spots hidden generics: a method taking a class-type union param (e.g. New_|MethodCall) but declaring a broader single return type (their shared parent Node), then returning that very param. In such a case a @template generic narrows the return to the passed type instead of the broad parent.

Complements the existing PreferGenericTemplateOnSameUnionTypeRule, which only covers the case where both param and return are the same union. This one covers the broad-parent return.

Guards against false positives:

• skips when a @template is already present
• skips scalar unions
• skips when the method does not return the param
• skips when the return type is one of the union members (not a shared parent), e.g. a loop that reassigns the variable before returning

The rule found 3 real cases in the codebase, 2 of which are fixed here with @template generics:

• RemoveNullTagValueNodeRector::processVarTagNull()
• RemoveZeroBreakContinueRector::processVariableNum()

The third (SideEffectNodeDetector::resolveVariable()) is correctly skipped by the union-member guard.