fix: preserve HTTP status code in streamable HTTP client error messages

When the streamable HTTP transport receives a 4xx/5xx response it
currently reports a generic 'Server returned an error response' message,
discarding the actual HTTP status code and any response body.  This
makes it very hard to debug upstream issues (e.g. 401 vs 403 vs 502).

Include the HTTP status code and up to 200 chars of the response body
in the error message so callers get actionable diagnostics.

Fixes #2110

Author: g97iulio1609

src/mcp/client/streamable_http.py

@@ -276,7 +276,14 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
 
             if response.status_code >= 400:
                 if isinstance(message, JSONRPCRequest):
-                    error_data = ErrorData(code=INTERNAL_ERROR, message="Server returned an error response")
+                    try:
+                        response_text = (await response.aread()).decode(errors="replace")
+                    except Exception:
+                        response_text = ""
+                    error_data = ErrorData(
+                        code=INTERNAL_ERROR,
+                        message=f"HTTP {response.status_code}: {response_text[:200]}" if response_text else f"HTTP {response.status_code}",
+                    )
                     session_message = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
                     await ctx.read_stream_writer.send(session_message)
                 return