[Bug] Partial-update merge-function keeps stale row when the last message is a lone UPDATE_BEFORE (-U)

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Paimon version

1.2.0

### Compute Engine

flink 1.20

### Minimal reproduce step

## Create the source changelog table
```
CREATE TABLE testlog(
id STRING PRIMARY KEY NOT ENFORCED,
f1 STRING,
delete INT
) WITH (
'merge-engine' = 'deduplicate',
'changelog-producer' = 'lookup'
);

-- seed data
INSERT INTO testlog VALUES ('11', '11', 0), ('12', '12', 0);

-- update id=12 to be logically deleted
INSERT INTO testlog VALUES ('12', '12', 1), ('13', '13', 0);
```

## Query source table
```
SELECT * FROM testlog;

op id f1 delete
+I 11 11 0
+I 12 12 0
-U 12 12 0
+U 12 12 1
+I 13 13 0
```

## Query source table with filter
```
SELECT * FROM testlog WHERE delete = 0;

op id f1 delete
+I 11 11 0
+I 12 12 0
-U 12 12 0
+I 13 13 0
```
Note:
After applying the filter delete = 0, the +U 12 12 1 is completely dropped.
Consequently, the only message about id = 12 that reaches the downstream Paimon table is -U 12 12 0.

## Filter out logically-deleted rows and write into a partial-update table
```
CREATE TABLE testlog01 (
id STRING PRIMARY KEY NOT ENFORCED,
f1 STRING,
delete INT
) WITH (
'merge-engine' = 'partial-update',
'partial-update.remove-record-on-delete' = 'true',
'changelog-producer' = 'lookup'
);

INSERT INTO testlog01
SELECT * FROM testlog WHERE delete = 0;
```

## Query the target table
```
SELECT * FROM testlog01;

```

### What doesn't meet your expectations?

## Expected result
```
op id f1 delete
+I 11 11 0
+I 12 12 0
-D 12 12 0
+I 13 13 0
```

(id=12 should delete because its last message is -U and no +U reaches the sink)

## Actual result
```
op id f1 delete
+I 11 11 0
+I 12 12 0
-U 12 12 0
+U 12 12 0
+I 13 13 0
```
The stale row id=12 is still present, breaking data correctness.

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start with the provided Flink 1.20 SQL reproduction and trace how the partial-update table handles the filtered changelog containing a lone -U for id=12. Inspect the partial-update merge behavior and verify the target output against the expected result, where id=12 is deleted rather than retained as a stale row.

Written by the indexing model from the issue text.