mcp: don't omit required fields in ImageContent and AudioContent (#95)

Update `ImageContent` and `AudioContent` to use an inline wire format
to ensure that required fields are not omitted during JSON marshaling,
matching TypeScript schema requirements. This follows the TextContent
approach.

- `ImageContent` requires `data` and `mimeType` fields.
- `AudioContent` requires `data` and `mimeType` fields.

mcp/content_test.go

@@ -45,6 +45,14 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"image","mimeType":"image/png","data":"YTFiMmMz"}`,
 		},
+		{
+			&mcp.ImageContent{MIMEType: "image/png", Data: []byte{}},
+			`{"type":"image","mimeType":"image/png","data":""}`,
+		},
+		{
+			&mcp.ImageContent{Data: []byte("test")},
+			`{"type":"image","mimeType":"","data":"dGVzdA=="}`,
+		},
 		{
 			&mcp.ImageContent{
 				Data:        []byte("a1b2c3"),
@@ -61,6 +69,14 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"audio","mimeType":"audio/wav","data":"YTFiMmMz"}`,
 		},
+		{
+			&mcp.AudioContent{MIMEType: "audio/wav", Data: []byte{}},
+			`{"type":"audio","mimeType":"audio/wav","data":""}`,
+		},
+		{
+			&mcp.AudioContent{Data: []byte("test")},
+			`{"type":"audio","mimeType":"","data":"dGVzdA=="}`,
+		},
 		{
 			&mcp.AudioContent{
 				Data:        []byte("a1b2c3"),