Consolidate session ID propagation into existing ProcessImplStartAdvice

Remove separate ProcessBuilderSessionId instrumentation files and fold
env var injection into the existing ProcessImplStartAdvice. Adds
_DD_ROOT_JAVA_SESSION_ID to the child process environment map directly
in the ProcessImpl.start() hook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: khanayan123

dd-java-agent/instrumentation/java/java-lang/java-lang-1.8/src/main/java/datadog/trace/instrumentation/java/lang/ProcessBuilderSessionIdAdvice.java

@@ -1,13 +1,23 @@
 package datadog.trace.instrumentation.java.lang;
 
+import datadog.trace.api.Config;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
 import datadog.trace.bootstrap.instrumentation.api.AgentTracer;
 import datadog.trace.bootstrap.instrumentation.api.java.lang.ProcessImplInstrumentationHelpers;
+import java.util.Map;
 import net.bytebuddy.asm.Advice;
 
 class ProcessImplStartAdvice {
+  @SuppressWarnings("unchecked")
   @Advice.OnMethodEnter(suppress = Throwable.class)
-  public static AgentSpan beforeStart(@Advice.Argument(0) final String[] command) {
+  public static AgentSpan beforeStart(
+      @Advice.Argument(0) final String[] command,
+      @Advice.Argument(value = 1, readOnly = false) Map<String, String> environment) {
+    String rootSessionId = Config.get().getRootSessionId();
+    if (rootSessionId != null && environment != null) {
+      environment.put("_DD_ROOT_JAVA_SESSION_ID", rootSessionId);
+    }
+
     if (!ProcessImplInstrumentationHelpers.ONLINE) {
       return null;
     }

dd-java-agent/instrumentation/java/java-lang/java-lang-1.8/src/main/java/datadog/trace/instrumentation/java/lang/ProcessBuilderSessionIdInstrumentation.java

@@ -3,6 +3,7 @@ package datadog.trace.instrumentation.java.lang
 import datadog.trace.agent.test.InstrumentationSpecification
 import datadog.trace.agent.test.asserts.SpanAssert
 import datadog.trace.agent.test.utils.TraceUtils
+import datadog.trace.api.Config
 import datadog.trace.bootstrap.ActiveSubsystems
 
 import java.util.concurrent.TimeUnit
@@ -331,4 +332,17 @@ class ProcessImplInstrumentationSpecification extends InstrumentationSpecificati
     ['/does/not/exist/cmd', '--pass', 'abc', '--token=def'] | '["/does/not/exist/cmd","--pass","?","--token=?"]'
     ['/does/not/exist/md5', '-s', 'pony']                   | '["/does/not/exist/md5","?","?"]'
   }
+
+  void 'child process inherits root session ID'() {
+    when:
+    def command = ['/bin/sh', '-c', 'echo $_DD_ROOT_JAVA_SESSION_ID']
+    def builder = new ProcessBuilder(command)
+    Process p = builder.start()
+    def output = p.inputStream.text.trim()
+    def terminated = p.waitFor(5, TimeUnit.SECONDS)
+
+    then:
+    terminated
+    output == Config.get().getRootSessionId()
+  }
 }