chore: remove the dead integrationTest source set and task

[mlwelles]

Description

build.gradle declares an integrationTest source set and a matching Test task. Neither has ever run anything. Four independent reasons, each sufficient on its own:

1. The configured source directory, src/integration-test/java, does not exist. Nor does src/integration-test/resources.
2. task integrationTest(type: Test) never calls useTestNG(). It would default to JUnit 4 and run none of this project's TestNG tests even if sources appeared.
3. Its dependency configurations extend testCompile and testRuntime, both removed in Gradle 7. This project builds with Gradle 8.14.2.
4. Nothing wires the task into check, and CI runs ./gradlew build -i, which excludes it.

Removing it also drops four no-op tasks from ./gradlew tasks (compileIntegrationTestJava, extractIntegrationTestProto, generateIntegrationTestProto, processIntegrationTestResources) plus the integrationTest task itself. Their presence suggests a working unit/integration split exists, which sends anyone looking for one down a dead end.

What this does not do

Splitting the cluster-dependent tests out of src/test is still worth doing. 19 of the 23 test classes need a live cluster, so ./gradlew test cannot run without one — awkward now that the suite has genuinely server-free tests. That work needs scaffolding that functions, and it touches the CI workflow, so it belongs in its own change rather than being grafted onto a cleanup.

Verification

• ./gradlew build -x test succeeds.
• ./gradlew tasks --all no longer lists any integrationTest task; nothing else references the source set.
• Server-free tests still run (ExceptionMappingTest, RetryPolicyTest).

No source or public API changes.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable —
  n/a, this removes build configuration and adds no behavior

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}