fix: Resolve Maven classloader conflict and Gradle missing dependency

- Maven: Pre-resolve SCC plugin for ALL modules in test profile, not
  just 'producer', to avoid BasicAuthCache ClassCastException when
  later modules (producer_with_xml) trigger SNAPSHOT resolution
- Gradle: Add maven-core to buildscript classpath for
  producer_with_external_contracts to fix ClassNotFoundException for
  SettingsDecryptionRequest during copyContracts task

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: marcingrzejszczak

.github/workflows/maven.yml

@@ -24,6 +24,6 @@ jobs:
     - name: Pre-resolve SCC plugin to avoid classloader conflict
       run: |
         cd common && ./mvnw dependency:resolve-plugins -B && cd ..
-        ./mvnw dependency:resolve-plugins -Ptest -B -pl producer || true
+        ./mvnw dependency:resolve-plugins -Ptest -B || true
     - name: Build with Maven
       run: ./scripts/runMavenBuilds.sh

producer_with_external_contracts/build.gradle

@@ -1,3 +1,12 @@
+buildscript {
+	repositories {
+		mavenCentral()
+	}
+	dependencies {
+		classpath 'org.apache.maven:maven-core:3.9.6'
+	}
+}
+
 plugins {
 	id 'groovy'
 	id 'org.springframework.boot'