Test the schema warnings through a shared log-capture helper - #878

Fixes #867

Checklist

• Pull requests follows the contribution guide
• New or modified functionality is covered by tests

Description

We log a WARN for a bunch of schema problems we tolerate, but no test checked any of them. So a warning could disappear or change without anything failing.

UnmappedInputFieldsTest already had a withWarnings { ... } helper from #857 that attaches a logback ListAppender and returns the result along with the logged warnings. It's moved to TestUtils.kt as is, except it now listens on the graphql.kickstart.tools logger instead of the root one, so graphql-java's own warnings don't end up in the list.

The tests that already build these schemas now assert the warnings too:

• unused types: unused unions are reported by their definition and get the members of their extensions when included
• repeated enum values, interface fields and union members: repeats and extensions without a type keep building
• extensions of a different kind than their type: extensions of a different kind than the type are ignored
• a non-repeatable directive applied again by an extension: directives of extensions follow those of the type and are wired

allowUnimplementedResolvers had no test of its own, so MissingFieldResolverTest gets one for the option's warning and the "Missing resolver for field" one.

The lists are exact, so an extra warning fails the test as well as a missing one.

Test only, no library changes. mvn clean verify passes on JDK 25 and on JDK 11 against graphql-java 25.1.

The other warnings (unused dictionary entries and resolvers, JDK proxies, preferGraphQLResolver, setAccessible failures, the deprecated context ones) weren't in the issue and are still untested, but withWarnings works for those too.