[Bug] using Chain table group partition ,partition in snapshot but not in delta will not be read

### Search before asking

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

### Paimon version

i tried using chain group parition to solve my #7503 , but still failed read some data

### Compute Engine

spark 3.5.2

### Minimal reproduce step

using test case :
```
CREATE TABLE IF NOT EXISTS `default`.`chain_test` (
`t1` BIGINT COMMENT 't1',
`t2` BIGINT COMMENT 't2',
`t3` STRING COMMENT 't3'
) using paimon PARTITIONED BY (`region` STRING, `dt` STRING COMMENT 'dt')
TBLPROPERTIES (
'bucket-key' = 't1',
'primary-key' = 'region,dt,t1',
'partition.timestamp-pattern' = '$dt',
'partition.timestamp-formatter' = 'yyyyMMdd',
'chain-table.enabled' = 'true',
'bucket' = '2',
'merge-engine' = 'deduplicate',
'sequence.field' = 't2',
'chain-table.chain-partition-keys' = 'dt'
);

CALL sys.create_branch('default.chain_test', 'snapshot');
CALL sys.create_branch('default.chain_test', 'delta');

ALTER TABLE default.chain_test SET TBLPROPERTIES (
'scan.fallback-snapshot-branch' = 'snapshot',
'scan.fallback-delta-branch' = 'delta'
);

ALTER TABLE `default`.`chain_test$branch_snapshot` SET TBLPROPERTIES (
'scan.fallback-snapshot-branch' = 'snapshot',
'scan.fallback-delta-branch' = 'delta'
);

ALTER TABLE `default`.`chain_test$branch_delta` SET TBLPROPERTIES (
'scan.fallback-snapshot-branch' = 'snapshot',
'scan.fallback-delta-branch' = 'delta'
);
```
insert some data
```
-- 写入 delta 分支
SET spark.paimon.branch = delta;

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'CN', dt = '20250809')
VALUES (1, 1, '1'), (2, 1, '1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'CN', dt = '20250810')
VALUES (1, 2, '1-1'), (3, 1, '1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'CN', dt = '20250811')
VALUES (2, 2, '1-1'), (4, 1, '1');
VALUES (5, 2, '1-1'), (6, 2, '1-1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'US', dt = '20250809')
VALUES (11, 1, '1'), (12, 1, '1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'US', dt = '20250810')
VALUES (11, 2, '1-1'), (13, 1, '1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'US', dt = '20250811')
VALUES (12, 2, '1-1'), (14, 1, '1');

-- 写入 snapshot 分支
SET spark.paimon.branch = snapshot;

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'CN', dt = '20250810')
VALUES (1, 2, '1-1'), (2, 1, '1'), (3, 1, '1');

INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'US', dt = '20250810')
VALUES (11, 2, '1-1'), (12, 1, '1'), (13, 1, '1');

```
query
```
SELECT * FROM `default`.`chain_test` where dt = '20250811'
```

result
```
1 2 1-1 CN 20250811
2 2 1-1 CN 20250811
4 1 1 CN 20250811
3 1 1 CN 20250811
12 2 1-1 US 20250811
11 2 1-1 US 20250811
13 1 1 US 20250811
14 1 1 US 20250811
```
this is ok, why? because all partition are in all branch

if i insert a partition not in delta
```
SET spark.paimon.branch = snapshot;
INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'UK', dt = '20250810')
VALUES (1, 2, '1-1'), (2, 1, '1'), (3, 1, '1');
```
still got query result
```
1 2 1-1 CN 20250811
2 2 1-1 CN 20250811
4 1 1 CN 20250811
3 1 1 CN 20250811
12 2 1-1 US 20250811
11 2 1-1 US 20250811
13 1 1 US 20250811
14 1 1 US 20250811
```

### What doesn't meet your expectations?

when insert a partition not in delta ,and in snapshot
```
INSERT OVERWRITE TABLE `default`.`chain_test` PARTITION (region = 'UK', dt = '20250810')
VALUES (1, 2, '1-1'), (2, 1, '1'), (3, 1, '1');
```

query result should be
```
1 2 1-1 CN 20250811
2 2 1-1 CN 20250811
4 1 1 CN 20250811
3 1 1 CN 20250811
12 2 1-1 US 20250811
11 2 1-1 US 20250811
13 1 1 US 20250811
14 1 1 US 20250811
1 2 1-1 UK 20250811
2 1 1 UK 20250811
3 1 1 UK 20250811
```

or i try to change partition key order from `PARTITIONED BY (`region` STRING, `dt` STRING COMMENT 'dt')` to `PARTITIONED BY (`dt` STRING COMMENT 'dt',`region` STRING)`

when execute sql as above
result is
```
2 2 1-1 20250811 US
4 1 1 20250811 US
12 2 1-1 20250811 US
14 1 1 20250811 US
2 2 1-1 20250811 CN
4 1 1 20250811 CN

```

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by running the provided Spark SQL reproduction with chain-table partitioning and separate snapshot and delta branches, then compare reads for partitions present in only one branch. Trace the partition-selection behavior for `dt = '20250811'`; done means the query also returns the expected UK rows from the snapshot branch without changing results for CN and US.

Written by the indexing model from the issue text.