### What happened?

When proxying an external MCP server through the LiteLLM MCP gateway (`/<server>/mcp`), every `tools/call` fails with:

``` MCP mcp_server_tool_call - error: 'int' object is not subscriptable Traceback (most recent call last): File ".../litellm/proxy/_experimental/mcp_server/server.py", line 910, in mcp_server_tool_call File ".../litellm/proxy/_experimental/mcp_server/server.py", line 735, in _capture_host_progress_callback TypeError: 'int' object is not subscriptable ```

The upstream tool actually executes successfully — the crash is in the progress-callback capture path, so the successful result is discarded and the caller only sees an error.

### Root cause

In `litellm/proxy/_experimental/mcp_server/server.py`, `_capture_host_progress_callback` slices the progress token assuming it is a string:

```python host_token = getattr(host_ctx.meta, "progressToken", None) ... verbose_logger.debug(f"Host progressToken captured: {host_token[:8]}...") # line 735 ```

Per the MCP spec, a `progressToken` may be **either a string or an integer** (`string | number`). When the client (or downstream server) uses an **integer** progress token, `host_token[:8]` raises `Ty…