Prior to this commit, the auto-configured
`ExecutionGraphQlServiceTester` would use a default Jackson
`ObjectMapper` provided by the builder for its JSON
serialization/deserialization needs. Other testers, like the
`HttpGraphQlTester` are based on auto-configured components which
already use the auto-configured `ObjectMapper`.
This commit uses the newly introduced `encoder(Encoder<?>)` and
`decoder(Decoder<?>)` builder methods to configure custom JSON codecs
honoring the application configuration.
Closes gh-30646
e0ae1d3501