fix: make ApiSpecTask compatible with Gradle Configuration Cache

[grubmeshi]

Summary

Move objectMapper instantiation from a task instance field into the @TaskAction method to make OpenApi3Task, OpenApiTask, and PostmanTask compatible with Gradle's Configuration Cache.

Problem

Jackson's ObjectMapper contains non-serializable fields (StdDateFormat, ReentrantLock) that prevent Gradle from serializing the task graph into the configuration cache. This causes:

Configuration cache state could not be cached: field `_dateFormat` of
`com.fasterxml.jackson.databind.cfg.BaseSettings` bean found in field `objectMapper`
of task `:generateOpenApi3Raw` of type `com.epages.restdocs.apispec.gradle.OpenApi3Task`:
error writing value of type 'com.fasterxml.jackson.databind.util.StdDateFormat'

Every build that includes these tasks gets its configuration cache entry discarded, negating any caching benefit.

Fix

Since objectMapper is only used inside aggregateResourceModels(), it does not need to be a persistent field on the task instance. Moving it to a local variable inside the @TaskAction method avoids the serialization issue entirely.

Fixes #284