chore: raise ee-test JVM heap to 8G to avoid OOM under Spring Boot 4 (#3841)

## Problem

`ee-test:test` intermittently fails with `java.lang.OutOfMemoryError`
(Java heap), surfacing as flaky failures in whichever test class happens
to be loading a Spring context when the heap fills — e.g.
`QaCheckFinalizationTest`, `QaCheckRunnerServiceTest`. The tests pass in
isolation; only the full suite OOMs.

## Cause

The `ee-test` suite runs all ~984 tests in a single JVM with
`maxHeapSize = "4096m"`, and caches Spring test contexts
(`spring.test.context.cache` default cap = 32). ee-test has many
distinct `@SpringBootTest`/`@MockitoBean` configurations, so a large
number of distinct contexts get cached. Spring Boot 4 contexts are
heavier than SB3's, so 32 cached contexts now intermittently exceed the
4G heap — the `OutOfMemoryError` is thrown by the JVM (heap exhausted),
not an OS OOM-kill. Memory config was not changed by the SB4 migration;
the same 4G heap now holds heavier contexts.

## Fix

Raise the forked test-JVM heap `4096m → 8192m`. GitHub `ubuntu-24.04`
runners have ~16G, and during the test task only this one forked JVM
plus the gradle process and the Postgres/Redis containers are running,
so 8G is safe headroom with no OS memory pressure.

If OOM recurs even at 8G, the follow-up would be to cap
`spring.test.context.cache.maxSize` (fewer retained contexts) or shard
`ee-test` — but a heap bump is the minimal first step.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Increased the memory available during test execution to improve
reliability for memory-intensive test runs.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: bdshadow

ee/backend/tests/build.gradle

@@ -29,7 +29,7 @@ allOpen {
 
 tasks.named('test', Test) {
     useJUnitPlatform()
-    maxHeapSize = "4096m"
+    maxHeapSize = "8192m"
     rootProject.setTestRetry(it)
 }