Make generateJava cacheable and relocatable

[iuliiasobolevska]

Summary

• make GenerateJavaTask cacheable with relative schema inputs and lazy DirectoryProperty outputs
• preserve the existing generatedSourcesDir, schemaPaths, getOutputDir, and getExampleOutputDir DSL/JVM accessors as compatibility bridges
• disable cache storage and loading only when generated Jakarta annotations include timestamps
• add TestKit coverage for cross-workspace cache reuse, output equivalence, up-to-date behavior, input invalidation, and the timestamped-output opt-out

Validation

JAVA_HOME=/Users/isobolevska/.sdkman/candidates/java/17.0.19-zulu ./gradlew \
  :graphql-dgs-codegen-gradle:formatKotlinMain \
  :graphql-dgs-codegen-gradle:formatKotlinTest \
  :graphql-dgs-codegen-gradle:validatePlugins \
  :graphql-dgs-codegen-gradle:test --no-scan

BUILD SUCCESSFUL on Gradle 9.4.1. The cross-directory TestKit test reports FROM-CACHE; the unchanged local rerun reports UP-TO-DATE; changed scalar, boolean, map, list, and schema inputs execute the task again.

Adversarial review

An independent line-by-line review of commit c1963d11 found no correctness issues after verification:

• all three new TestKit tests and the complete Gradle plugin test suite pass
• mutation-testing doNotCacheIf to always return false makes the timestamped-annotation test fail because the second workspace goes FROM-CACHE, proving the opt-out test detects the regression
• validatePlugins --rerun passes without missing-annotation or overlapping-output warnings, including the two DirectoryProperty outputs and the @Internal compatibility accessors
• existing provider, file-collection, mutable-list and multimodule DSL compatibility fixtures pass
• the cross-workspace helper's output-root substitution was checked against the exact fixture text, ruling out accidental shared output paths

The review also confirmed that the self-assignment in init intentionally invokes the custom setter for the initialized default, flatMap(GenerateJavaTask::generatedSourcesDirectory) preserves lazy source-set wiring, and the cache predicate reads the final configured values at execution time.

The example output directory remains outside Gradle source-set wiring, as it was before this change; that pre-existing behavior is out of scope.

Benchmark

An external consumer fixture using a shared local cache executed generateJava in the first workspace and restored it FROM-CACHE in a second absolute workspace path. The generated output trees were byte-identical. End-to-end wall time was 19.51s for the initial included-build run and 0.88s for the warm cached run. This is directional because the first number includes daemon and included-build setup; the material result is that code generation is skipped entirely on the cache hit.