[java] Fix UnnecessaryCast false-negative in method calls

[zbynek]

Describe the PR

There is a false negative in the UnnecessaryCast rule when an object is cast just to call a method that's available on its declared type. Example:

class A {
    private Integer x;

    public Integer getX() {
        return x;
    }

    public static class B extends A { }

    public int compareTo(A arg) {
        return ((B) arg).getX().compareTo(getX()); // should be a violation
    }
}

We can simply call arg.getX() without casting it to B.

Related issues

This is somewhat similar to #3219, but in this PR I focused on the case when the cast is on methods's receiver, not its arguments.

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)

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Compared to main:
This changeset changes 0 violations,
introduces 6 new violations, 0 new errors and 0 new configuration errors,
removes 0 violations, 0 errors and 0 configuration errors.

Regression Tester Report

(comment created at 2025-09-17 22:17:46+00:00 for b7456fe)

[oowekyala]

Note that this is in principle the same problem as #5948, you need to check that the selected overload is the same. I would like the logic to be shared if possible.

[adangel]

Thanks!