Address PR feedback: include rootSessionId in logs, respect runtimeIdEnabled, fix test

- Add rootSessionId to Config.toString() for tracer log visibility
- Gate getRootSessionId() behind runtimeIdEnabled like getRuntimeId()
- Force environment map initialization in test to fix test_inst failures

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

Author: khanayan123

dd-java-agent/instrumentation/java/java-lang/java-lang-1.8/src/test/groovy/datadog/trace/instrumentation/java/lang/ProcessImplInstrumentationSpecification.groovy

@@ -337,6 +337,7 @@ class ProcessImplInstrumentationSpecification extends InstrumentationSpecificati
     when:
     def command = ['/bin/sh', '-c', 'echo $_DD_ROOT_JAVA_SESSION_ID']
     def builder = new ProcessBuilder(command)
+    builder.environment() // force lazy environment map initialization so advice can inject
     Process p = builder.start()
     def output = p.inputStream.text.trim()
     def terminated = p.waitFor(5, TimeUnit.SECONDS)

internal-api/src/main/java/datadog/trace/api/Config.java

@@ -3130,7 +3130,7 @@ public String getRuntimeId() {
   }
 
   public String getRootSessionId() {
-    return RuntimeIdHolder.rootSessionId;
+    return runtimeIdEnabled ? RuntimeIdHolder.rootSessionId : "";
   }
 
   public Long getProcessId() {
@@ -5873,6 +5873,9 @@ public String toString() {
         + ", runtimeId='"
         + getRuntimeId()
         + '\''
+        + ", rootSessionId='"
+        + getRootSessionId()
+        + '\''
         + ", runtimeVersion='"
         + runtimeVersion
         + ", apiKey="