fix: omit null version id for suspended buckets

Keep null as the stored object version ID when bucket versioning is suspended, but omit x-amz-version-id from the PutObject response to match Amazon S3.

Update the existing versioning integration coverage to distinguish the PutObject response from the null version returned by ListObjectVersions, and verify empty response headers are not rendered.

Fixes #2164

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

s3api/controllers/base_test.go

@@ -198,9 +198,10 @@ func TestSetResponseHeaders(t *testing.T) {
 		headers map[string]*string
 	}
 	tests := []struct {
-		name     string
-		args     args
-		expected map[string]string
+		name       string
+		args       args
+		expected   map[string]string
+		unexpected []string
 	}{
 		{
 			name: "should not set if map is nil",
@@ -215,7 +216,7 @@ func TestSetResponseHeaders(t *testing.T) {
 				headers: map[string]*string{
 					"x-amz-checksum-algorithm": utils.GetStringPtr("crc32"),
 					"x-amz-meta-key":           utils.GetStringPtr("meta_key"),
-					"x-amz-mp-size":            utils.GetStringPtr(""),
+					"x-amz-version-id":         utils.GetStringPtr(""),
 					"something":                nil,
 				},
 			},
@@ -223,6 +224,7 @@ func TestSetResponseHeaders(t *testing.T) {
 				"x-amz-checksum-algorithm": "crc32",
 				"x-amz-meta-key":           "meta_key",
 			},
+			unexpected: []string{"x-amz-version-id", "something"},
 		},
 	}
 	for _, tt := range tests {
@@ -236,6 +238,9 @@ func TestSetResponseHeaders(t *testing.T) {
 					assert.Equal(t, val, string(v))
 				}
 			}
+			for _, key := range tt.unexpected {
+				assert.Empty(t, ctx.Response().Header.Peek(key))
+			}
 		})
 	}
 }