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[java] Fix #6409: UnnecessaryCast false positive for anonymous class (#7193)

PMD types an anonymous class creation like `new Foo() {}` as `Foo`,
so the rule saw `(Foo) new Foo() {}` as an identity cast and reported
it. The type of the expression is the anonymous class, a strict subtype
of `Foo`. In a lambda returned to a generic method such as
`stream.map(i -> (Foo) new Foo() {}).toList()` the cast decides the
inferred type, and removing it does not compile.

The rule now uses the anonymous class type for the operand, so these
casts follow the same checks as casts of any other subtype.

Co-authored-by: ybai08 <271969568+ybai08@users.noreply.github.com>

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