Revert "fix: return null when error type is null"

This reverts commit e665b6e.

Author: diegomarquezp

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/ErrorTypeUtil.java

@@ -119,12 +119,12 @@ enum ErrorType {
   // Requirement source: go/clo:product-requirements-v1
   public static String extractErrorType(@Nullable Throwable error) {
     if (error == null) {
-      // No information about the error; This attribute should not be recorded.
-      return null;
+      // No information about the error; we default to INTERNAL.
+      return ErrorType.INTERNAL.toString();
     }
 
     // 1. Unwrap standard wrapper exceptions if present
-    Throwable realError = unwrapError(error);
+    Throwable realError = getRealCause(error);
 
     // 2. Attempt to extract specific error type from the main exception
     String specificError = extractKnownErrorType(realError);
@@ -143,7 +143,7 @@ public static String extractErrorType(@Nullable Throwable error) {
   }
 
   /** Unwraps standard execution wrappers to find the real cause. */
-  private static Throwable unwrapError(Throwable t) {
+  private static Throwable getRealCause(Throwable t) {
     if (t.getCause() == null) {
       return t;
     }

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/tracing/ErrorTypeUtilTest.java

@@ -54,7 +54,8 @@ class ErrorTypeUtilTest {
 
   @Test
   void testExtractErrorType_null() {
-    assertThat(ErrorTypeUtil.extractErrorType(null)).isNull();
+    assertThat(ErrorTypeUtil.extractErrorType(null))
+        .isEqualTo(ErrorTypeUtil.ErrorType.INTERNAL.toString());
   }
 
   @Test

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/tracing/SpanTracerTest.java

@@ -439,8 +439,10 @@ void testAttemptFailed_internalFallback_nullError() {
     // For an anonymous inner class Throwable, getSimpleName() is empty string,
     // which triggers the
     // fallback
-    verify(span, never())
-        .setAttribute(eq(ObservabilityAttributes.ERROR_TYPE_ATTRIBUTE), anyString());
+    verify(span)
+        .setAttribute(
+            ObservabilityAttributes.ERROR_TYPE_ATTRIBUTE,
+            ErrorTypeUtil.ErrorType.INTERNAL.toString());
     verify(span).end();
   }