mcp: render blank text field in TextContent instead of omitempty (#91)

The text field is required, but current implementation omits the text
field when it is a blank string.

Some clients fail to parse the response if the text field is omitted. A
blank text result is a valid MCP response.

Claude desktop error message:
```
ClaudeAiToolResultRequest.content.0.text.text: Field required
```

The solution is to return the field even when it is blank by removing
`omitempty`.

mcp/content_test.go

@@ -22,6 +22,14 @@ func TestContent(t *testing.T) {
 			&mcp.TextContent{Text: "hello"},
 			`{"type":"text","text":"hello"}`,
 		},
+		{
+			&mcp.TextContent{Text: ""},
+			`{"type":"text","text":""}`,
+		},
+		{
+			&mcp.TextContent{},
+			`{"type":"text","text":""}`,
+		},
 		{
 			&mcp.TextContent{
 				Text:        "hello",