Extract the OpenTelemetry agent test wiring into a convention plugin #6378

The Spring Boot 2, 3 and 4 modules each copy the same setup to run their OpenTelemetry agent tests in a child JVM: the agentFixture source set, the otelAgentJar configuration, runAgentFixture, and the integrationTestOtelAgentWithoutAutoInit task. The copies are identical; only the dependency lists differ. Each module also has to know that a plain project dependency on sentry-opentelemetry-agent gives the -dontuse jar, and that the real agent needs Bundling.SHADOWED and a non-transitive configuration.

Move the identical parts into one convention plugin in build-logic (e.g. io.sentry.otel-agent-tests), next to ChildJvmTestArguments:

• create the agentFixture source set
• create otelAgentJar and add the dependency on sentry-opentelemetry-agent
• register the integrationTestOtelAgent suite wired to the child JVM, and the WithoutAutoInit task

The modules keep only their Boot-specific dependencies.

Also fix the comment in sentry-opentelemetry-agent/build.gradle.kts. The runtimeElements jar isn't just a by-product: it holds only AgentMarker, and the Boot modules' unit tests rely on it. The agent itself is shadowRuntimeElements.

Do this after the Boot 4 (#6317) and Boot 2 agent tests land, so the in-flight PRs don't need rewriting.