Add reusable Gradle helpers for derived JVM test suite variants

Is your feature request related to a problem? Please describe.

Instrumentation build scripts repeatedly map over testing.suites.withType(JvmTestSuite::class) to register Experimental, StableSemconv, and BothSemconv test variants. Each map repeats task naming, test class directories, runtime classpath, metadata configuration, and sometimes source-suite JVM settings and enabled state. The repetition is easy to get wrong when a source suite adds another JVM argument or system property.

Describe the solution you'd like

Add a typed helper to the shared Gradle conventions for registering a derived Test task per JvmTestSuite. The helper should bind the source suite's classes and classpath, optionally inherit its explicit JVM arguments, system properties, and enabled state, and retain a configuration block for variant-specific settings.

val stableSemconvSuites = registerTestSuiteVariants("StableSemconv") {
  otelConfig("otel.semconv-stability.opt-in=database")
}

val experimentalSuites = registerTestSuiteVariants(
  suffix = "Experimental",
  inheritSourceSettings = true,
) {
  otelConfig("otel.instrumentation.kafka.experimental-span-attributes=true")
}

The implementation must not use Test.copyTo(): Gradle flattens source jvmArgumentProviders into ordinary arguments, and repository conventions later attach providers to the derived task, which can add the javaagent twice and lose provider input tracking.

Start by migrating the uniform per-suite maps. Keep specialized task filters and multi-variant combinations in their build scripts until the helper proves it can represent them clearly.

Describe alternatives you've considered

Keep the existing maps in each module. This is explicit, but it preserves substantial boilerplate and makes source-suite inheritance a per-file responsibility. A helper specialized only for semconv tasks would be smaller, but the same registration pattern is already needed for experimental variants.

Additional context

PR #19738 introduced experimental variants for custom JVM suites and documented the safe source-setting inheritance pattern in .github/agents/knowledge/gradle-conventions.md and .github/agents/knowledge/testing-experimental-flags.md. This issue is the follow-up for consolidating that pattern rather than expanding the current PR.

Tip

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Read .github/agents/knowledge/gradle-conventions.md and .github/agents/knowledge/testing-experimental-flags.md, then locate the repeated testing.suites.withType(JvmTestSuite::class) maps in the instrumentation build scripts. Done means uniform per-suite maps use the shared helper with safe source-setting inheritance, while specialized filters and multi-variant combinations remain in their scripts without using Test.copyTo().