`_add_token_transfer` silently ignores `is_approved` when merging transfers for the same account

### Description

When `_add_token_transfer()` is called multiple times for the same `(token_id, account_id)` pair, the transfer amount is accumulated correctly, but the `is_approved` flag is never updated in the accumulation branch.

This causes approved transfers to silently lose their approval state depending on call order.

Example:

```python
tx.add_token_transfer(token_a, sender, 500)
# is_approved=False (default)

tx.add_approved_token_transfer(token_a, sender, 200)
# expected accumulated transfer to become approved

### Steps to reproduce

1. Create a transaction.
2. Add a normal token transfer for a `(token_id, account_id)` pair.
3. Add an approved token transfer for the same `(token_id, account_id)` pair.
4. Inspect the accumulated transfer object.

Example:

```python
tx.add_token_transfer(token_a, sender, 500)
tx.add_approved_token_transfer(token_a, sender, 200)
```

Observed result:

```python
is_approved == False
```

Expected result:

```python
is_approved == True
```

### Additional context

Root cause appears to be inside `_add_token_transfer()` where the accumulation branch updates `amount` and `expected_decimals`, but never updates `is_approved`.

Current logic:

```python
for transfer in self.token_transfers[token_id]:
if transfer.account_id == account_id:
transfer.amount += amount
transfer.expected_decimals = expected_decimals
return
```

Suggested fix:

```python
transfer.is_approved = is_approved
```

before returning from the accumulation branch.

This can affect allowance-based transfer flows where regular and approved transfers are combined in the same transaction.

### Hedera network

_No response_

### Version

v0.2.5

### Operating system

None