Fix test infrastructure for cross-platform stability

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

Author: jbachorik

ddprof-test/src/test/java/com/datadoghq/profiler/context/TagContextTest.java

@@ -161,6 +161,9 @@ private void checkTagValues(ContextSetter contextSetter, String contextAttribute
 
     @Override
     protected String getProfilerCommand() {
-        return "wall=1ms,filter=0,attributes=tag1;tag2;tag3";
+        // Use cstack=default explicitly: this test covers context-tagging correctness,
+        // not stack-walking mode.  CSTACK_VM is the global default but is unreliable
+        // on certain CI runners (musl x64), so we pin to a stable mode here.
+        return "wall=1ms,filter=0,cstack=default,attributes=tag1;tag2;tag3";
     }
 }

ddprof-test/src/test/java/com/datadoghq/profiler/junit/CStackInjector.java

@@ -50,9 +50,17 @@ public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContex
             // zing is a bit iffy when using anything but 'no' for cstack
             return Stream.of(new ParameterizedTestContext("no", retryCount));
         } else {
-            return Stream.of(valueSource.strings()).
-                    filter(CStackInjector::isModeSafe).
-                    map(param -> new ParameterizedTestContext(param, retryCount));
+            List<String> safeValues = Stream.of(valueSource.strings())
+                    .filter(CStackInjector::isModeSafe)
+                    .collect(Collectors.toList());
+            if (safeValues.isEmpty()) {
+                // No mode passed the filter (e.g. J9 with a {"vm","vmx"} @ValueSource).
+                // Fall back to the first declared value so the test can reach
+                // isPlatformSupported() and skip cleanly rather than failing with
+                // PreconditionViolationException (JUnit 5 requires ≥1 invocation context).
+                safeValues = Collections.singletonList(valueSource.strings()[0]);
+            }
+            return safeValues.stream().map(param -> new ParameterizedTestContext(param, retryCount));
         }
     }