Gradle plugin injects the Grails BOM into every configuration, defeating defaultDependencies (breaks core Gradle jacoco plugin)

### Expected Behavior

Applying core Gradle's `jacoco` plugin to a Grails app works as it does on Grails 7.x (verified on 7.2.3): the `jacocoAgent` and `jacocoAnt` configurations resolve `org.jacoco:org.jacoco.agent` / `org.jacoco:org.jacoco.ant` via the jacoco plugin's `defaultDependencies { }` hooks, and `./gradlew test` runs with coverage.

### Actual Behaviour

On Grails 8.0.0-M3 through M6 (re-verified at each milestone), `./gradlew test` fails:
```
Execution failed for task ':test'.
> Expected configuration ':jacocoAgent' to contain exactly one file, however, it contains no files.
```
`./gradlew dependencies --configuration jacocoAgent` shows the configuration contains only the injected `org.apache.grails:grails-hibernate5-bom` platform. Gradle's `defaultDependencies { }` only fires on an otherwise-empty configuration (gradle/gradle#11209 documents that a platform-only configuration does not count as empty), so the jacoco plugin's defaults never run and the agent/ant jars are never added. The mechanism breaks any plugin that populates a configuration via `defaultDependencies`, not just jacoco.

Likely origin: `GrailsGradlePlugin.applyGrailsBom()` (introduced with the platform migration in #15467, applied in `afterEvaluate`) adds the BOM platform to every configuration; #16009 already adjusted its ordering. Suggested fix: scope the injection away from configurations owned by other plugins (or add the platform via `withDependencies` only where version management is actually needed).

Workaround we use:
```groovy
dependencies {
jacocoAgent "org.jacoco:org.jacoco.agent:${jacoco.toolVersion}"
jacocoAnt "org.jacoco:org.jacoco.ant:${jacoco.toolVersion}"
}
```

### Steps To Reproduce

1. Grails 8.0.0-M6 app (M3, M4, M5 also reproduce).
2. Add `id 'jacoco'` to the plugins block.
3. `./gradlew test`
4. Observe the `jacocoAgent` failure; `./gradlew dependencies --configuration jacocoAgent` shows only the BOM platform.

### Environment Information

Linux, JDK 21 (Temurin 21.0.12), Gradle 8.14. Grails 8.0.0-M3..M6, grails-hibernate5 BOM. Works on Grails 7.2.3

### Example Application

https://github.com/davidbairdala/grails8-bom-defaultdeps-repro

### Version

8.0.0-M6 (present since 8.0.0-M3)

Start in GrailsGradlePlugin.applyGrailsBom(), especially its afterEvaluate ordering, and compare the behavior introduced by #15467 and adjusted by #16009. Reproduce with the linked Grails 8 BOM example by applying the jacoco plugin, then inspect ./gradlew dependencies --configuration jacocoAgent and ./gradlew test. Done means jacocoAgent and jacocoAnt resolve their default dependencies and tests run with coverage without the workaround.

Written by the indexing model from the issue text.