fix: ensure developmentOnly configuration exists with BOM platform

The Spring Dependency Management plugin previously applied BOM version
constraints globally across all configurations. With the switch to
Gradle's native platform(), only configurations that explicitly receive
the platform dependency get managed versions.

Use maybeCreate to guarantee the developmentOnly configuration always
exists, even if plugin ordering changes or Spring Boot is not applied,
and add the grails-bom platform to it so dependencies like
spring-boot-devtools can be declared without a version.

Assisted-by: Claude Code <Claude@Claude.ai>

Author: jamesfredley

grails-gradle/plugins/src/main/groovy/org/grails/gradle/plugin/core/GrailsGradlePlugin.groovy

@@ -386,6 +386,14 @@ ${importStatements}
 
         project.dependencies.add('implementation', project.dependencies.platform(bomCoordinates))
 
+        // Ensure the developmentOnly configuration exists and receives BOM platform
+        // constraints. Spring Boot's plugin normally creates this configuration, but
+        // it does not extend from implementation so platform version constraints do
+        // not flow to it automatically. Using maybeCreate guarantees the configuration
+        // is available even if plugin ordering changes or Spring Boot is not applied.
+        project.configurations.maybeCreate('developmentOnly')
+        project.dependencies.add('developmentOnly', project.dependencies.platform(bomCoordinates))
+
         project.afterEvaluate {
             BomManagedVersions managedVersions = BomManagedVersions.resolve(project, bomCoordinates)
             if (managedVersions.hasOverrides()) {