fix(storage): keep token amounts exact instead of truncating to uint64

TokenRecord.Amount was uint64 while the backing column is
amount NUMERIC(78, 0), sized for a 256-bit value. AppendToken narrowed the
quantity with big.Int.Uint64(), which returns the low 64 bits with no error
for anything larger, so for a token whose quantity exceeds 2^64-1 the amount
column silently received a wrapped value while the authoritative hex quantity
column stayed correct.

Widen the field to *big.Int and insert its exact decimal representation,
following the pattern already used for movement and transaction records.
StoreToken now refuses a nil amount (the column is NOT NULL) and one wider
than maxAmountBits, so a value the column cannot hold surfaces as an error
instead of a row whose two amount columns disagree. No consumer relied on the
uint64 type; the only call sites were the test fixtures updated here.

Note that only Postgres can hold such a value: SQLite gives a NUMERIC column
NUMERIC affinity and converts an integer literal too large for int64 to REAL.
With this change that fails loudly on read rather than corrupting silently.
The exact round-trip case is therefore wired into the Postgres suite, while
the write-time validation case runs on every backend.

Fixes #2021

Signed-off-by: AkramBitar <akram@il.ibm.com>

Author: AkramBitar

token/services/selector/testutils/test_cases.go

@@ -9,6 +9,7 @@ package testutils
 import (
 	"context"
 	"fmt"
+	"math/big"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -173,7 +174,7 @@ func (m *enhancedManager) UpdateTokens(deleted []*token.ID, added []token.Unspen
 				LedgerMetadata: []byte{},
 				Quantity:       t.Quantity,
 				Type:           t.Type,
-				Amount:         0,
+				Amount:         big.NewInt(0),
 				Owner:          true,
 				Auditor:        false,
 				Issuer:         false,

token/services/storage/db/dbtest/tokenlock.go

@@ -7,6 +7,7 @@ SPDX-License-Identifier: Apache-2.0
 package dbtest
 
 import (
+	"math/big"
 	"testing"
 	"time"
 
@@ -79,7 +80,7 @@ func TestFully(t *testing.T, tokenDB driver3.TokenStore, tokenLockDB driver3.Tok
 		LedgerMetadata: []byte{}, // Empty metadata
 		Quantity:       "0x64",   // 100 in hex
 		Type:           "USD",
-		Amount:         100,
+		Amount:         big.NewInt(100),
 		Owner:          true,
 	}
 	err = tokenTx.StoreToken(ctx, tokenRecord, []string{"owner1"})