mcp: preserve transport errors in Write error chain (#888)

mcp: preserve transport errors in Write error chain

Custom RoundTrippers that return typed errors (e.g. for auth failures)
have those errors flattened to strings by `%v`, making `errors.As`
unusable for callers inspecting transport-level errors.

This is inconsistent with the OAuth handler path (line 1807) which
already uses `%w` to preserve errors for client inspection.

Relates to #579

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

mcp/streamable.go

@@ -1795,14 +1795,14 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
 			// Failure to set headers means that the request was not sent.
 			// Wrap with ErrRejected so the jsonrpc2 connection doesn't set writeErr
 			// and permanently break the connection.
-			return nil, nil, fmt.Errorf("%s: %w: %v", requestSummary, jsonrpc2.ErrRejected, err)
+			return nil, nil, fmt.Errorf("%s: %w: %w", requestSummary, jsonrpc2.ErrRejected, err)
 		}
 		resp, err := c.client.Do(req)
 		if err != nil {
 			// Any error from client.Do means the request didn't reach the server.
 			// Wrap with ErrRejected so the jsonrpc2 connection doesn't set writeErr
 			// and permanently break the connection.
-			err = fmt.Errorf("%s: %w: %v", requestSummary, jsonrpc2.ErrRejected, err)
+			err = fmt.Errorf("%s: %w: %w", requestSummary, jsonrpc2.ErrRejected, err)
 		}
 		return req, resp, err
 	}