LoggingMessageNotification.data is String, so the client fails on object data (e.g. from FastMCP servers) #1180

Bug description
The MCP spec allows any JSON value in notifications/message params.data (type unknown). The SDK declares LoggingMessageNotification.data as String. When a server sends an object, the client cannot parse the notification. It logs an ERROR and drops the log message. The tool call itself still works.

FastMCP sends data as an object for every ctx.info() / ctx.log(): {"msg": "...", "extra": null}. So any SDK client calling a FastMCP tool that logs will hit this.

ERROR i.m.spec.McpClientSession : Error handling notification: Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`)
 (through reference chain: io.modelcontextprotocol.spec.McpSchema$LoggingMessageNotification["data"])

Environment

• MCP Java SDK 2.0.1 (also 2.0.0)
• Spring AI 2.0.0, Java 21
• Server: FastMCP 3.3.1, Streamable HTTP

Steps to reproduce

1. Run the FastMCP server below.
2. Call the hello tool with McpSyncClient.

Expected behavior
The notification is parsed and passed to the logging consumers. data should accept any JSON value, as the spec says.

Minimal Complete Reproducible example
Server:

from fastmcp import FastMCP, Context

mcp = FastMCP("repro")

@mcp.tool
async def hello(ctx: Context) -> str:
    await ctx.info("working")
    return "ok"

mcp.run(transport="streamable-http", port=8000)

Client:

var client = McpClient.sync(
        HttpClientStreamableHttpTransport.builder("http://localhost:8000").endpoint("/mcp").build())
    .build();
client.callTool(McpSchema.CallToolRequest.builder("hello").build()); // returns "ok", but logs the ERROR above

Possible fix that does not break the API: keep String, but turn a non-string data into its JSON text when parsing.