test: add Spark S3 integration regression for issue #8234 (#8249)

* test: add Spark S3 regression integration test for issue 8234

* Update test/s3/spark/setup_test.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

test/s3/spark/issue_8234_repro_test.go

@@ -0,0 +1,76 @@
+package spark
+
+import (
+	"strings"
+	"testing"
+)
+
+func setupSparkIssue8234Env(t *testing.T) *TestEnvironment {
+	t.Helper()
+
+	env := NewTestEnvironment()
+	if !env.dockerAvailable {
+		t.Skip("Docker not available, skipping Spark S3 integration test")
+	}
+
+	env.StartSeaweedFS(t)
+	t.Cleanup(func() { env.Cleanup() })
+
+	createObjectBucket(t, env, "test")
+	env.startSparkContainer(t)
+	return env
+}
+
+func TestSparkS3AppendIssue8234Regression(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping Spark integration test in short mode")
+	}
+
+	env := setupSparkIssue8234Env(t)
+
+	script := `
+import sys
+import pyspark.sql.types as T
+
+target = "s3a://test/test_table/"
+sc = spark.sparkContext
+df = sc.parallelize([1, 2, 3]).toDF(T.IntegerType())
+
+df.write.format("parquet").mode("append").save(target)
+print("FIRST_APPEND_OK")
+
+try:
+    df.write.format("parquet").mode("append").save(target)
+    print("SECOND_APPEND_OK")
+    spark.stop()
+    sys.exit(0)
+except Exception as err:
+    message = str(err)
+    print("SECOND_APPEND_FAILED")
+    print(message)
+    if "Copy Source must mention the source bucket and key" in message:
+        print("ISSUE_8234_REGRESSION")
+        spark.stop()
+        sys.exit(4)
+    spark.stop()
+    sys.exit(2)
+`
+
+	code, output := runSparkPyScript(t, env.sparkContainer, script, env.s3Port)
+	if code != 0 {
+		if strings.Contains(output, "ISSUE_8234_REGRESSION") {
+			t.Fatalf("issue #8234 regression detected; output:\n%s", output)
+		}
+		t.Fatalf("Spark script exited with code %d; output:\n%s", code, output)
+	}
+
+	if !strings.Contains(output, "FIRST_APPEND_OK") {
+		t.Fatalf("expected first append success marker in output, got:\n%s", output)
+	}
+	if !strings.Contains(output, "SECOND_APPEND_OK") {
+		t.Fatalf("expected second append success marker in output, got:\n%s", output)
+	}
+	if strings.Contains(output, "Copy Source must mention the source bucket and key") {
+		t.Fatalf("unexpected issue #8234 error in output:\n%s", output)
+	}
+}