Fix reserveMethodNames to reserve each supplied name

[junhyeong9812]

Overview

GeneratedClass.reserveMethodNames(String...) reserves a set of method names so that
generated methods will not collide with them.

Problem

Inside the per-name loop, the whole varargs array was passed to MethodName.of(...) instead
of the current element:

for (String reservedMethodName : reservedMethodNames) {
    String generatedName = generateSequencedMethodName(MethodName.of(reservedMethodNames)); // whole array
    Assert.state(generatedName.equals(reservedMethodName), ...);
}

MethodName.of(String...) joins all parts into a single camel-case name, so
reserveMethodNames("apply", "test") produced "applyTest", and the per-element check
Assert.state("applyTest".equals("apply")) failed with IllegalStateException. Single-name
calls worked only by accident (a one-element array round-trips to the same name). The only
in-tree caller reserves a single name, so the multi-name path was never exercised.

Fix

Pass the loop variable so that each supplied name is reserved individually. A regression test
in GeneratedClassTests reserves two names.

Note on intent

This fix assumes reserveMethodNames is meant to reserve each supplied name individually.
The per-element assertion Assert.state(generatedName.equals(reservedMethodName)) and the
javadoc ("a set of reserved method names") both indicate this. If the intent was instead to
reserve a single combined name, the loop and the per-element assertion would be inconsistent
and a different fix would be appropriate — happy to adjust if I have misread the intent.