fix: update error msg for MPT float inputs; fix #796 (#813)

---------

Co-authored-by: Omar Khan <[email protected]>

Author: ckeshava

xrpl/core/binarycodec/types/amount.py

@@ -125,7 +125,9 @@ def verify_mpt_value(mpt_value: str) -> None:
     """
     # Contains no decimal point
     if not _contains_decimal(mpt_value):
-        raise XRPLBinaryCodecException(f"{mpt_value} is an invalid MPT amount.")
+        raise XRPLBinaryCodecException(
+            f"{mpt_value} is a decimal value. Please use string type to represent it."
+        )
 
     decimal = None
     try:

xrpl/models/amounts/mpt_amount.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import Dict, Union
+from typing import Dict
 
 from typing_extensions import Self
 
@@ -24,7 +24,7 @@ class MPTAmount(BaseModel):
     :meta hide-value:
     """
 
-    value: Union[str, int, float] = REQUIRED  # type: ignore
+    value: str = REQUIRED  # type: ignore
     """
     This field is required.